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Having been a ground for various topological fermionic phases, the family of ZrSiS-type 111 materials has been under experimental and theoretical investigations. Within this family of materials, the subfamily LnSbTe (Ln = lanthanide elements) is gaining interests in recent times as the strong correlation effects and magnetism arising from the 4f electrons of the lanthanides can provide an important platform to study the linking between topology, magnetism, and correlation. In this paper, we report the systematic study of the electronic structure of SmSbTe - a member of the LnSbTe subfamily - by utilizing angle-resolved photoemission spectroscopy in conjunction with first-principles calculations, transport, and magnetic measurements. Our experimental results identify multiple Dirac nodes forming the nodal-lines along the G- X and Z- R directions in the bulk Brillouin zone (BZ) as predicted by our theoretical calculations. A surface Dirac-like state that arises from the square net plane of the Sb atoms is also observed at the X point of the surface BZ. Our study highlights SmSbTe as a promising candidate to understand the topological electronic structure of LnSbTe materials., 7 pages, 4 figures

We investigate theoretically the superconducting state of the undoped Fe-based superconductor ThFeAsN. Using input from ab initio calculations, we solve the Fermi-surface based, multichannel Eliashberg equations for Cooper-pair formation mediated by spin and charge fluctuations, and by the electron-phonon interaction (EPI). Our results reveal that spin fluctuations alone, when coupling only hole-like with electron-like energy bands, can account for a critical temperature ${T}_{c}$ up to $\ensuremath{\sim}7.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ with an ${s}_{\ifmmode\pm\else\textpm\fi{}}$-wave superconducting gap symmetry, which is a comparatively low ${T}_{c}$ with respect to the experimental value ${T}_{c}^{\mathrm{exp}}=30\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Other combinations of interaction kernels (spin, charge, electron-phonon) lead to a suppression of ${T}_{c}$ due to phase frustration of the superconducting gap. We qualitatively argue that the missing ingredient to explain the gap magnitude and ${T}_{c}$ in this material might be the first-order correction to the EPI vertex. In the noninteracting state this correction adopts a form supporting the ${s}_{\ifmmode\pm\else\textpm\fi{}}$ gap symmetry, in contrast to EPI within Migdal's approximation, i.e., EPI without vertex correction, and therefore it enhances tendencies arising from spin fluctuations.

Recently, kagome materials have become an engrossing platform to study the interplay among symmetry, magnetism, topology, and electron correlation. The latest works on RMn6Sn6 (R = rare earth metal) compounds have illustrated that this family could be intriguing to investigate various physical phenomena due to large spin-orbit coupling and strong magnetic ordering. However, combined transport and spectroscopic studies in RMn6Sn6 materials are still limited. Here, we report magnetic, magneto-transport, and angle-resolved photoemission spectroscopy measurements of a kagome magnet ErMn6Sn6 that undergoes antiferromagnetic (TN = 345 K) to ferrimagnetic (TC = 68 K) phase transitions in the presence of field. We observe large anomalous and topological Hall effects serving as transport signatures of the nontrivial Berry curvature. The isothermal magnetization exhibits strong anisotropic nature and the topological Hall effect of the compound depends on the critical field of metamagnetic transition. Our spectroscopic results complemented by theoretical calculations show the multi-orbital kagome fermiology. This work provides new insight into the tunability and interplay of topology and magnetism in a kagome magnet., Comment: 18 pages, 18 figures; Supplementary Information included, To be published in PRB

Ultrashort light pulses can selectively excite charges, spins and phonons in materials, providing a powerful approach for manipulating their properties. Here we use femtosecond laser pulses to coherently manipulate the electron and phonon distributions, and their couplings, in the charge density wave (CDW) material 1T-TaSe$_2$. After exciting the material with a short light pulse, spatial smearing of the electrons launches a coherent lattice breathing mode, which in turn modulates the electron temperature. This indicates a bi-directional energy exchange between the electrons and the strongly-coupled phonons. By tuning the laser excitation fluence, we can control the magnitude of the electron temperature modulation, from ~ 200 K in the case of weak excitation, to ~ 1000 K for strong laser excitation. This is accompanied by a switching of the dominant mechanism from anharmonic phonon-phonon coupling to coherent electron-phonon coupling, as manifested by a phase change of $\pi$ in the electron temperature modulation. Our approach thus opens up possibilities for coherently manipulating the interactions and properties of quasi-2D and other quantum materials using light., Comment: 15 pages, 4 figures

Using trARPES, we observe that the CDW amplitude mode in 1T-TaSe2 can coherently modulate the electron temperature. Coherent electron-phonon coupling mechanisms are proposed to explain the electron temperature oscillations at different pump fluences.

We elucidate the structure of assemblies formed by three conjugated molecules with very similar chemical structure, differing only by two fluorine atoms, on an Ag 111 surface to gain insight into the intricate interplay of attractive and repulsive interactions that govern the self assembly of molecules on a metal surface. With scanning tunneling microscopy we observe substantially different self assembled structures for sub monolayers of parasexiphenyl 6P and its two partially fluorinated derivatives ortho 2F 6P and meta 2F 6P see Fig. 1 , which we can fully rationalize only with state of the art density functional theory modeling. By deliberate discriminating all involved interactions, that is, electrostatic due to permanent charges, polarization and dispersion, as well as hydrogen bonding, we can provide new insights for advanced self assembly strategies. We demonstrate that fluorination at selected positions of conjugated molecules provides for sufficiently strong, yet nonrigid, H F bonding capability to circumvent dominating repulsive interactions at very low molecular surface coverage. This enables the realization and steering of individual and stable nanoscale molecular assemblies as well as their study, without the need to approach monolayer coverage, which could substantially alter the obtained structure. Furthermore, the insight provided here helps to understand how fluorine substitution in conjugated molecules and polymers contributes to thin film and bulk structures, which, in turn will enable realizing organic electronic materials with superior optical and charge transport properties for electronic and optoelectronic applications

Microscopic understanding of molecular adsorption on catalytic surfaces is crucial for controlling the activity and selectivity of chemical reactions. However, for complex molecules, the adsorption process is very system-specific and there is a clear need to elaborate systematic understanding of important factors that determine catalytic functionality. Here, we investigate the binding of eight molecules, including seven alkoxides and one carboxylate, on the Au(111) and Au(110) surfaces. Our density-functional theory calculations including long-range van der Waals interactions demonstrate the significant role of these “weak” noncovalent forces on the adsorption structures, energetics, and relative adsorbate stabilities. Interestingly, the binding energy trends are insensitive to the surface structure. Instead, the adsorption stability depends strongly on the structural and chemical characteristics of the molecules: linear vs branching configurations, number of unsaturated C–C bonds, bidentate adsorption, a...

Adsorption energies of chemisorbed molecules on inorganic solids usually scale linearly with molecular size and are well described by additive scaling laws. However, much less is known about scaling laws for physisorbed molecules. Our temperature-programmed desorption experiments demonstrate that the adsorption energy of acenes (benzene to pentacene) on the Au(111) surface in the limit of low coverage is highly nonadditive with respect to the molecular size. For pentacene, the deviation from an additive scaling of the adsorption energy amounts to as much as 0.7 eV. Our first-principles calculations explain the observed nonadditive behavior in terms of anisotropy of molecular polarization stemming from many-body electronic correlations. The observed nonadditivity of the adsorption energy has implications for surface-mediated intermolecular interactions and the ensuing on-surface self-assembly. Thus, future coverage-dependent studies should aim to gain insights into the impact of these complex interactions on the self-assembly of π-conjugated organic molecules on metal surfaces.

Enhancing the selectivity of catalytic processes has potential for substantially increasing the sustainability of chemical production. Herein, we establish relationships between reaction selectivity and molecular structure for a homologous series of key intermediates for oxidative coupling of alcohols on gold using a combination of experiment and theory. We establish a scale of binding for molecules with different alkyl structures and chain lengths and thereby demonstrate the critical nature of noncovalent van der Waals interactions in determining the selectivity by modulating the stability of key reaction intermediates bound to the surface. The binding hierarchy is the same for Au(111) and Au(110), which demonstrates a relative lack of sensitivity to the surface structure. The hierarchy of binding established in this work provides guiding principles for predicting how molecular structure affects the competition for binding sites more broadly. Besides the nature of the primary surface-molecule bonding, three additional factors that affect the stabilities of the reactive intermediates are clearly established: (1) the number of C atoms in the alkyl chain, (2) the presence of C-C bond unsaturation, and (3) the degree of branching of the alkyl group of the adsorbed molecules. We suggest that this is a fundamental principle that is generally applicable to a broad range of reactions on metal catalysts.

The ultrafast relaxation dynamics of a nonequilibrium phonon gas towards thermal equilibrium involves many-body collisions that cannot be properly described by perturbative approaches. Here, we develop a nonperturbative method to elucidate the microscopic mechanisms underlying the decay of laser-excited coherent phonons in the presence of electron-hole pairs, which so far are not fully understood. Our theory relies on ab initio molecular dynamics simulations on laser-excited potential-energy surfaces. Those simulations are compared with runs in which the laser-excited coherent phonon is artificially deoccupied. We apply this method to antimony and show that the decay of the A1g phonon mode at low laser fluences can be accounted mainly to three-body down-conversion processes of an A1g phonon into acoustic phonons. For higher excitation strengths, however, we see a crossover to a four-phonon process, in which two A1g phonons decay into two optical phonons.

By means of first principles calculations, we studied the possibility of manipulating structural properties of different materials via excitation with intense femtosecond laser or extreme ultraviolet (XUV) pulses. For silicon and boron-nitride nanotubes, we performed ab initio molecular dynamics simulations using the code CHIVES, developed in our group, to describe their laser-induced structural dynamics. For both materials, we determined the damage thresholds. We also investigated the structural response of magnesium and copper to ultrashort XUV excitation. For this purpose, we performed frozen-phonon calculations based on all-electron density functional theory and allowed the possibility of core-hole excitation. We found that Cu undergoes bond hardening and Mg bond softening upon creation of core holes and hot electrons, where we defined the bond strength by the vibrational frequencies.

Ultrashort optical pulses can be used both to create fundamental quasiparticles in crystals and to change their properties. In noble metals, femtosecond lasers induce bond hardening, but little is known about its origin and consequences. Here we simulate ultrafast laser excitation of silver at high fluences. We compute laser-excited potential-energy surfaces by all-electron ab initio theory and analyze the resulting quantum lattice dynamics. We also consider incoherent lattice heating due to electron-phonon interactions using the generalized two-temperature model. We find phonon hardening, which we attribute to the excitation of $s$ electrons. We demonstrate that this may result in phonon vacuum squeezed states with an optimal squeezing factor of $\ensuremath{\sim}0.001$ at the L-point longitudinal mode. This finding implies that ultrafast laser-induced bond hardening may be used as a tool to manipulate the quantum state of opaque materials, where, so far, the squeezing of phonons below the zero-point motion has only been realized in transparent crystals by a different mechanism. On the basis of our finding, we further propose a method for directly measuring bond hardening.

Intense ultrashort extreme ultraviolet (XUV) pulses can be used to create warm dense matter in the laboratory, which then develops to a plasma state. So far, however, it is unknown, whether this transition occurs via heat transfer from hot electrons to cold atoms or nonthermally due to a lattice instability. Here we computed the response of the phonon spectra of copper and silver to the presence of XUV-excited core holes and core holes together with very hot electrons. We found that the average interatomic bonds become stronger in the warm dense state. We discuss why these findings support the above-mentioned heat transfer scenario.