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- ItemAtomic and molecular suite of R-matrix codes for ultrafast dynamics in strong laser fields and electron/positron scattering(Bristol : IOP Publ., 2020) Wragg, J.; Benda, J.; MaÅ¡Ãn, Z.; Armstrong, G.S.J.; Clarke, D.D.A.; Brown, A.C.; Ballance, C.; Harvey, A.G.; Houfek, K.; Sunderland, A.; Plummer, M.; Gorfinkiel, J.D.; Van Der Hart, H.We describe and illustrate a number of recent developments of the atomic and molecular ab initio R-matrix suites for both time-dependent calculations of ultrafast laser-induced dynamics and time-independentcalculations of photoionization and electron scattering. © 2019 Published under licence by IOP Publishing Ltd.
- ItemNormal State O 17 NMR Studies of Sr2RuO4 under Uniaxial Stress(College Park, Md. : APS, 2019) Luo, Yongkang; Pustogow, A.; Guzman, P.; Dioguardi, A.P.; Thomas, S.M.; Ronning, F.; Kikugawa, N.; Sokolov, D.A.; Jerzembeck, F.; Mackenzie, A.P.; Hicka, C.W.; Bauer, E.D.; Mazin, I.I.; Brown, S.E.The effects of uniaxial compressive stress on the normal state O17 nuclear-magnetic-resonance properties of the unconventional superconductor Sr2RuO4 are reported. The paramagnetic shifts of both planar and apical oxygen sites show pronounced anomalies near the nominal a-axis strain μaaμv that maximizes the superconducting transition temperature Tc. The spin susceptibility weakly increases on lowering the temperature below T≃10 K, consistent with an enhanced density of states associated with passing the Fermi energy through a van Hove singularity. Although such a Lifshitz transition occurs in the γ band formed by the Ru dxy states hybridized with in-plane O pπ orbitals, the large Hund's coupling renormalizes the uniform spin susceptibility, which, in turn, affects the hyperfine fields of all nuclei. We estimate this "Stoner" renormalization S by combining the data with first-principles calculations and conclude that this is an important part of the strain effect, with implications for superconductivity. © 2019 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the »https://creativecommons.org/licenses/by/4.0/» Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
- ItemExperimental Observation of Dirac Nodal Links in Centrosymmetric Semimetal TiB2(College Park, MD : American Physical Society, 2018) Liu, Z.; Lou, R.; Guo, P.; Wang, Q.; Sun, S.; Li, C.; Thirupathaiah, S.; Fedorov, A.; Shen, D.; Liu, K.; Lei, H.; Wang, S.The topological nodal-line semimetal state, serving as a fertile ground for various topological quantum phases, where a topological insulator, Dirac semimetal, or Weyl semimetal can be realized when the certain protecting symmetry is broken, has only been experimentally studied in very few materials. In contrast to discrete nodes, nodal lines with rich topological configurations can lead to more unusual transport phenomena. Utilizing angle-resolved photoemission spectroscopy and first-principles calculations, here, we provide compelling evidence of nodal-line fermions in centrosymmetric semimetal TiB2 with a negligible spin-orbit coupling effect. With the band crossings just below the Fermi energy, two groups of Dirac nodal rings are clearly observed without any interference from other bands, one surrounding the Brillouin zone (BZ) corner in the horizontal mirror plane σh and the other surrounding the BZ center in the vertical mirror plane σv. The linear dispersions forming Dirac nodal rings are as wide as 2 eV. We further observe that the two groups of nodal rings link together along the Γ-K direction, composing a nodal-link configuration. The simple electronic structure with Dirac nodal links mainly constituting the Fermi surfaces suggests TiB2 as a remarkable platform for studying and applying the novel physical properties related to nodal-line fermions.
- ItemObservation of orbital order in the van der Waals material 1T−TiSe2(College Park, MD : APS, 2022) Peng, Yingying; Guo, Xuefei; Xiao, Qian; Li, Qizhi; Strempfer, Jörg; Choi, Yongseong; Yan, Dong; Luo, Huixia; Huang, Yuqing; Jia, Shuang; Janson, Oleg; Abbamonte, Peter; van den Brink, Jeroen; van Wezel, JasperBesides magnetic and charge order, regular arrangements of orbital occupation constitute a fundamental order parameter of condensed matter physics. Even though orbital order is difficult to identify directly in experiments, its presence was firmly established in a number of strongly correlated, three-dimensional Mott insulators. Here, reporting resonant x-ray-scattering experiments on the layered van der Waals compound 1T-TiSe2, we establish that the known charge density wave in this weakly correlated, quasi-two-dimensional material corresponds to an orbital ordered phase. Our experimental scattering results are consistent with first-principles calculations that bring to the fore a generic mechanism of close interplay between charge redistribution, lattice displacements, and orbital order. It demonstrates the essential role that orbital degrees of freedom play in TiSe2, and their importance throughout the family of correlated van der Waals materials.
- ItemMechanical properties and twin boundary drag in Fe-Pd ferromagnetic shape memory foils-experiments and ab initio modeling(Bristol : IOP, 2011) Claussen, I.; Mayr, S.G.We report on vibrating reed measurements combined with density functional theory-based calculations to assess the elastic and damping properties of Fe-Pd ferromagnetic shape memory alloy splats. While the austenite-martensite phase transformation is generally accompanied by lattice softening, a severe modulus defect and elevated damping behavior are characteristic of the martensitic state. We interpret the latter in terms of twin boundary motion between pinning defects via partial 'twinning' dislocations. Energy dissipation is governed by twin boundary drag, primarily due to lattice imperfections, as concluded from the temperature dependence of damping and related activation enthalpies.
- ItemAb initio based study of finite-temperature structural, elastic and thermodynamic properties of FeTi(Amsterdam [u.a.] : Elsevier, 2014) Zhu, L.-F.; Friák, M.; Udyansky, A.; Ma, D.; Schlieter, A.; Kühn, U.; Eckert, J.; Neugebauer, J.We employ density functional theory (DFT) to calculate pressure dependences of selected thermodynamic, structural and elastic properties as well as electronic structure characteristics of equiatomic B2 FeTi. We predict ground-state single-crystalline Young's modulus and its two-dimensional counterpart, the area modulus, together with homogenized polycrystalline elastic parameters. Regarding the electronic structure of FeTi, we analyze the band structure and electronic density of states. Employing (i) an analytical dynamical matrix parametrized in terms of elastic constants and lattice parameters in combination with (ii) the quasiharmonic approximation we then obtained free energies, the thermal expansion coefficient, heat capacities at constant pressure and volume, as well as isothermal bulk moduli at finite temperatures. Experimental measurements of thermal expansion coefficient complement our theoretical investigation and confirm our theoretical predictions. It is worth mentioning that, as often detected in other intermetallics, some materials properties of FeTi strongly differ from the average of the corresponding values found in elemental Fe and Ti. These findings can have important implications for future materials design of new intermetallic materials.
- ItemThe electronic structure of ϵ-Ga2O3(Melville, NY : AIP Publ., 2019) Mulazzi, M.; Reichmann, F.; Becker, A.; Klesse, W.M.; Alippi, P.; Fiorentini, V.; Parisini, A.; Bosi, M.; Fornari, R.The electronic structure of ε-Ga2O3 thin films has been investigated by ab initio calculations and photoemission spectroscopy with UV, soft, and hard X-rays to probe the surface and bulk properties. The latter measurements reveal a peculiar satellite structure in the Ga 2p core level spectrum, absent at the surface, and a core-level broadening that can be attributed to photoelectron recoil. The photoemission experiments indicate that the energy separation between the valence band and the Fermi level is about 4.4 eV, a valence band maximum at the Γ point and an effective mass of the highest lying bands of – 4.2 free electron masses. The value of the bandgap compares well with that obtained by optical experiments and with that obtained by calculations performed using a hybrid density-functional, which also reproduce well the dispersion and density of states.
- ItemSpin Hall effect emerging from a noncollinear magnetic lattice without spin-orbit coupling(Bristol : Institute of Physics Publishing, 2018) Zhang, Y.; Železný, J.; Sun, Y.; Van Den Brink, J.; Yan, B.The spin Hall effect (SHE), which converts a charge current into a transverse spin current, has long been believed to be a phenomenon induced by spin-orbit coupling. Here, we identify an alternative mechanism to realize the intrinsic SHE through a noncollinear magnetic structure that breaks the spin rotation symmetry. No spin-orbit coupling is needed even when the scalar spin chirality vanishes, different from the case of the topological Hall effect and topological SHE reported previously. In known noncollinear antiferromagnetic compounds Mn3X (X = Ga, Ge, and Sn), for example, we indeed obtain large spin Hall conductivities based on ab initio calculations.
- ItemComprehensive scan for nonmagnetic Weyl semimetals with nonlinear optical response(London : Nature Publishing Group, 2020) Xu, Q.; Zhang, Y.; Koepernik, K.; Shi, W.; van den Brink, J.; Felser, C.; Sun, Y.First-principles calculations have recently been used to develop comprehensive databases of nonmagnetic topological materials that are protected by time-reversal or crystalline symmetry. However, owing to the low symmetry requirement of Weyl points, a symmetry-based approach to identifying topological states cannot be applied to Weyl semimetals (WSMs). To date, WSMs with Weyl points in arbitrary positions are absent from the well-known databases. In this work, we develop an efficient algorithm to search for Weyl points automatically and establish a database of nonmagnetic WSMs with Weyl points near the Fermi level based on the experimental non-centrosymmetric crystal structures in the Inorganic Crystal Structure Database (ICSD). In total, 46 Weyl semimetals were discovered to have nearly clean Fermi surfaces and Weyl points within 300 meV of the Fermi level. Nine of them are chiral structures which may exhibit the quantized circular photogalvanic effect. In addition, the nonlinear optical response is studied and the giant shift current is explored. Besides nonmagnetic WSMs, our powerful tools can also be used in the discovery of magnetic topological materials.
- ItemCoexistence of Superconductivity and Charge Density Waves in Tantalum Disulfide : Experiment and Theory(College Park, Md. : APS, 2020) Kvashnin, Y.; VanGennep, D.; Mito, M.; Medvedev, S.A.; Thiyagarajan, R.; Karis, O.; Vasiliev, A.N.; Eriksson, O.; Abdel-Hafiez, M.The coexistence of charge density wave (CDW) and superconductivity in tantalum disulfide (2H-TaS2) at low temperature is boosted by applying hydrostatic pressures to study both vibrational and magnetic transport properties. Around Pc, we observe a superconducting dome with a maximum superconducting transition temperature Tc=9.1 K. First-principles calculations of the electronic structure predict that, under ambient conditions, the undistorted structure is characterized by a phonon instability at finite momentum close to the experimental CDW wave vector. Upon compression, this instability is found to disappear, indicating the suppression of CDW order. The calculations reveal an electronic topological transition (ETT), which occurs before the suppression of the phonon instability, suggesting that the ETT alone is not directly causing the structural change in the system. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods. While a d wave and single-gap BCS prediction cannot describe the lower critical field Hc1 data, the temperature dependence of the Hc1 can be well described by a single-gap anisotropic s-wave order parameter. © 2020 authors. Published by the American Physical Society.