Fe substitution in URu 2 Si 2 : Singlet magnetism in an extended Doniach phase diagram
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Assessment of exchange-correlation functionals on oxygen vacancy formation energies of metal oxides
Oxygen vacancies are a common defect in oxides and play vital roles in many technological applications such as oxygen separation, catalytic reactors, solid oxide fuel cells, and solar thermochemical water splitting. The oxygen vacancy formation energy is directly related to the oxide reduction enthalpy and the vacancy concentration and, therefore, is a key quantity underlying these applications. The complexity of measuring oxygen vacancy formation energies experimentally suggests the utility of calculations based on density functional theory (DFT). However, the calculated results are strongly dependent on the exchange-correlation functionals and other parameters such as the Hubbard U . In this work, we compare the performance of the meta-GGA strongly constrained and appropriately normed (SCAN) functional and the commonly used semilocal generalized gradient approximation (GGA) functionals with experimental values of structural parameters, band gaps, magnetic structures, and oxygen vacancy formation energies for six representative oxides, i.e., CaMnO 3 , SrMnO 3 , LaMnO 3 , YMnO 3 , LaFeO 3 , and CeO 2 . Further, our results show that SCAN usually has better agreement with the experimental lattice constants and band gaps and larger magnetic moments than the commonly used GGA functionals. Although SCAN overestimates the oxygen vacancy formation energies of transition metal oxides and therefore requires unusually large Hubbard U values to reproduce the experimental reduction enthalpies, it does predict the correct oxygen vacancy formation energy of CeO 2 , which challenges the commonly used GGA and hybrid functionals. Our results underscore the challenges that exist in describing these complex oxides by DFT and may shed light on developing more accurate exchange-correlation functionals for oxygen vacancy formation energy calculations.
36 MATERIALS SCIENCE↗
Valence transition and termination-dependent surface states in the topological Kondo semimetal YbPtBi
Here, the Yb-terminated and Bi-terminated (111) surface electronic structure of topological Kondo semimetal YbPtBi is investigated using both density-functional-theory (DFT) -based calculations and angle-resolved photoemission spectroscopy (ARPES). The cleavage plane is found to be between Yb-layers and Bi-layers in both experiment and theory, and the broken inversion symmetry ensures the Yb-terminated surface does not mix with the Bi-terminated surface. The ARPES results at the Bi-terminated surface are similar to those reported for YPtBi or LuPtBi, and they can be well explained using DFT calculations assuming trivalent Yb atoms. In contrast, at the Yb-terminated surface, a trivalent to divalent transition and reduced hybridization with conduction electrons for the topmost Yb atoms are observed, as a result of reduced bonding with Bi-atoms. In addition, the Fermi arc features induced by the triply degenerate-points, which are missing at the Bi-terminated surface, can be identified at the Yb-terminated surface. Our study demonstrates unambiguously the reconstruction of surface electronic structure due to the valence transition of the lanthanide element in a Kondo lattice system.
36 MATERIALS SCIENCE↗
Signature of nodal topology in nonlinear quantum transport across junctions in Weyl and multi-Weyl semimetals
We investigate quantum transport through a rectangular potential barrier in Weyl semimetals (WSMs) and multi-Weyl semimetals (MSMs), within the framework of Landauer-Büttiker formalism. Here, our study uncovers the role of nodal topology imprinted in the electric current and the shot noise. We find that, in contrast to the finite odd-order conductance and noise power, the even-order contributions vanish at the nodes. Additionally, depending on the topological charge ($J$), the linear conductance ($G_1$) scales with the Fermi energy ($E_F$) as $G^{E_F>U}_{1}$ $∝$ $E^{2/J}_{F}$, $U$ being the barrier height. We demonstrate that the $E_F$ dependence of the second-order conductance and shot noise power could quite remarkably distinguish an MSM from a WSM depending on the band topology, and may induce several smoking gun experiments in nanostructures made out of WSMs and MSMs. Analyzing shot noise and Fano factor, we show that the transport across the rectangular barrier follows the sub-Poissonian statistics. Interestingly, we obtain universal values of Fano factor at the nodes unique to their topological charges. The universality for a fixed $J$, however, indicates that only a fixed number of open channels participate in the transport through evanescent waves at the nodes. The proposed results can serve as a potential diagnostic tool to identify different topological systems in experiments.
36 MATERIALS SCIENCE↗
Orbital-selective correlations and renormalized electronic structure in LiFeAs
Multiorbital physics is important to both the correlation physics and topological behavior of quantum materials. LiFeAs is a prototype iron pnictide suitable for in-depth investigation of this issue. Its electronic structure is strikingly different from the prediction of the noninteracting description. Here, a multiorbital Hubbard model is studied using a U(1) slave-spin theory. We demonstrate a mechanism for a substantial change to the Fermi surface, namely, orbital selectivity of the energy-level renormalization cooperating with its counterpart in quasiparticle spectral weight. Using this effect, we show how the dominating features of the electronic structure in LiFeAs are understood by the local correlations alone. In conclusion, our results set the stage to understand the origins and nature of both the unconventional superconductivity and likely electronic topology in this prototype iron pnictide and, more generally, reveal a remarkable degree of universality out of the seemingly complex multiorbital building blocks across a broad range of strongly correlated superconductors.
36 MATERIALS SCIENCE↗
Transient magnetoelastic coupling in CrSBr
Recent research has revealed remarkable properties of the two-dimensional (2D) van der Waals layered crystal CrSBr, which is both a semiconductor and an A-type antiferromagnet. Here, in this study, we show the role of strong magnetoelastic coupling in the generation and propagation of coherent magnons in CrSBr. Time- and spatially resolved magneto-optical Kerr effect microscopy reveals two time-varying transient strain fields induced by out-of-plane transverse and in-plane longitudinal lattice displacements. These transient strain fields launch coherent wavepackets of magnons, optical and acoustic, at 24.6 ± 0.8 and 33.4 ± 0.5GHz, respectively. These findings suggest mechanisms for controlling and manipulating coherent magnons from distinct magnetoelastic couplings in this 2D van der Waals magnetic semiconductor.
36 MATERIALS SCIENCE↗
Characteristic terahertz emissions induced by optically excited collective orbital modes
Here, we study the generation of collective orbital modes, their evolution, and the characteristic nonlinear optical response induced by them in a photoinduced orbital-ordered correlated oxide using real-time simulations based on an interacting multiband tight-binding (TB) model. The d-d optical transitions under femtoseconds light-pulse in an orbital-ordered state excite collective orbital modes, also known as “orbitons”. Consistently incorporating electronic interactions and the interplay between charge, spin, and lattice degrees of freedom in the TB-model provides a clearer understanding of how these factors influence the generation and evolution of collective orbital modes. The dynamics of Jahn-Teller vibrational modes in the photoinduced state modify the intersite orbital interaction, which further amplifies these orbital modes. In the presence of weak ferroelectricity, the excitation of collective orbital modes induces a strong THz oscillatory photocurrent, which is long-lived. This suggests an alternative way to experimentally detect low-energy collective modes through THz-emission studies in the photoinduced state. Our study also elucidates that quasiparticle dynamics in improper ferroelectric oxides can be exploited to achieve highly interesting and nontrivial optoelectronic properties.
36 MATERIALS SCIENCE↗
Second dome of superconductivity in YBa 2 Cu 3 O 7 at high pressure
Evidence is growing that a second dome of high-T c superconductivity can be accessed in the cuprates by increasing the doping beyond the first dome. Here, in this study, we use ab initio methods without invoking any free parameters, such as the Hubbard U, to reveal that pressure could turn YBa 2 Cu 3 O 7 into an ideal candidate for second dome superconductivity, displaying the predicted signature of strongly hybridized d x 2 -y 2 and d z 2 orbitals. Notably, pressure is found to induce a phase transition replacing the antiferromagnetic phases with an orbitally degenerate d-d phase. Our study suggests that the origin of the second dome is correlated with the oxygen-hole fraction in the CuO 2 planes and the collapse of the pseudogap phase.
36 MATERIALS SCIENCE↗
Approximate Hessian for accelerating ab initio structure relaxation by force fitting
We present a method to approximate the Hessian matrix of the Born-Oppenheimer energy landscape by using a simple force field model whose parameters are fitted to on-the-flight ab initio results. The inversed Hessian matrix is used as the preconditioner of conjugate gradient algorithms to speed up the atomic structure relaxation, resulting in a speedup factor of 2 to 5 on systems of bulk, slab, sheets, and atomic clusters. Because the force field model employed is simple and general, the parameter fitting is straightforward; the method is applicable to a variety of complicated systems for minimum structure relaxation. In the metal cluster new structure search, the new method yields better structures than the one obtained before with conventional algorithms.
36 MATERIALS SCIENCE↗
Reversible ratchet effects for vortices in conformal pinning arrays
A conformal transformation of a uniform triangular pinning array produces a structure called a conformal crystal which preserves the sixfold ordering of the original lattice but contains a gradient in the pinning density. Here we use numerical simulations to show that vortices in type-II superconductors driven with an ac drive over gradient pinning arrays produce the most pronounced ratchet effect over a wide range of parameters for a conformal array, while square gradient or random gradient arrays with equivalent pinning densities give reduced ratchet effects. In the conformal array, the larger spacing of the pinning sites in the direction transverse to the ac drive permits easy funneling of interstitial vortices for one driving direction, producing the enhanced ratchet effect. In the square array, the transverse spacing between pinning sites is uniform, giving no asymmetry in the funneling of the vortices as the driving direction switches, while in the random array, there are numerous easy-flow channels present for either direction of drive. We find multiple ratchet reversals in the conformal arrays as a function of vortex density and ac amplitude, and correlate the features with a reversal in the vortex ordering, which is greater for motion in the ratchet direction. In conclusion, the enhanced conformal pinning ratchet effect can also be realized for colloidal particles moving over a conformal array, indicating the general usefulness of conformal structures for controlling the motion of particles.
36 MATERIALS SCIENCE↗
Helicoidal graphene nanoribbons: Chiraltronics
Here, we present a calculation of the effective geometry-induced quantum potential for the carriers in graphene shaped as a helicoidal nanoribbon. In this geometry the twist of the nanoribbon plays the role of an effective transverse electric field in graphene and this is reminiscent of the Hall effect. But, this effective electric field has a different sign for the two isospin states and translates into a mechanism to separate the two chiral species on the opposing rims of the nanoribbon. Finally, isospin transitions are expected with the emission or absorption of microwave radiation which could be adjusted to be in the THz region.
36 MATERIALS SCIENCE↗
Phase transformations in Ln 2 O 3 materials irradiated with swift heavy ions
Phase transformations induced in the cubic C-type lanthanide sesquioxides, Ln 2 O 3 (Ln = Sm, Gd, Ho, Tm, and Lu), by dense electronic excitation are investigated. The structural modifications resulting from exposure to beams of 185 MeV Xe and 2246 MeV Au ions are characterized using synchrotron x-ray diffraction and Raman spectroscopy. The formation of a B-type polymorph, an X-type nonequilibrium phase, and an amorphous phase are observed. The specific phase formed and the transformation rate show dependence on the material composition, as well as the ion beam mass and energy. Atomistic mechanisms for these transformations are determined, indicating that formation of the B-type phase results from the production of anti-Frenkel defects and the aggregation of anion vacancies into planar clusters, whereas formation of the X-type and amorphous phases requires extensive displacement of both anions and cations. Here, the observed variations in phase behavior with changing lanthanide ionic radius and deposited electronic energy density are related to the energetics of these transformation mechanisms.
36 MATERIALS SCIENCE↗
Ultrafast relaxation dynamics in BiFeO 3 /YBa 2 Cu 3 O 7 bilayers
The temperature dependence of the relaxation dynamics in the bilayer thin film heterostructure composed of multiferroic BiFeO 3 (BFO) and superconducting YBa 2 Cu 3 O 7 (YBCO) grown on (001) SrTiO 3 substrate is studied by time-resolved pump-probe technique, and compared with that of pure YBCO thin film grown under the same growth conditions. The superconductivity of YBCO is found to be retained in the heterostructure. We observe a speeding up of the YBCO recombination dynamics in the superconducting state of the heterostructure, and attribute it to the presence of weak ferromagnetism at the BFO/YBCOinterface as observed inmagnetization data. An extension of the Rothwarf-Taylor model is used to fit the ultrafast dynamics of BFO/YBCO, that models an increased quasiparticle occupation of the ferromagnetic interfacial layer in the superconducting state of YBCO.
36 MATERIALS SCIENCE↗
Electronic Correlation and Magnetism in the Ferromagnetic Metal Fe 3 GeTe 2
Motivated by the search for design principles of rare-earth-free strong magnets, we present a study of electronic structure and magnetic properties of the ferromagnetic metal Fe3GeTe2 within local density approximation (LDA) of the density functional theory, and its combination with dynamical mean-field theory (DMFT). For comparison to these calculations, we have measured magnetic and thermodynamic properties as well as X-ray magnetic circular dichroism and the photoemission spectrum of single crystal Fe3GeTe2. We find that the experimentally determined Sommerfeld coefficient is enhanced by an order of magnitude with respect to the LDA value. This enhancement can be partially explained by LDA+DMFT. Additionally, the inclusion of dynamical electronic correlation effects provides the experimentally observed magnetic moments, and the spectral density is in better agreement with photoemission data. Lastly, these results establish the importance of electronic correlations in this ferromagnet.
36 MATERIALS SCIENCE↗
Pinched hysteresis loop in defect-free ferroelectric materials
In addition to the single polarization-versus-electric field hysteresis loop that is characteristic of ferroelectrics and the double hysteresis loop that is known to occur in antiferroelectrics, a third kind of polarization-versus-electric field function has been reported in several systems. This third kind is commonly termed the “pinched” loop due to its unusual shape, and is typically believed to originate from the pinning of domain walls interacting with defects. In this work, using an atomistic effective Hamiltonian scheme, we demonstrate that such a belief has to be broadened since our simulations also yield pinched loops in defect-free ferroelectric materials, as a result of the occurrence of intermediate modulated phases exhibiting an inhomogeneous dipolar pattern leading to the coexistence of both ferroelectric and antiferroelectric orders.
36 MATERIALS SCIENCE↗
Vortices, skyrmions, and chirality waves in frustrated Mott insulators with a quenched periodic array of impurities
Finite-Q magnetic instabilities are rather common in frustrated magnets. When the magnetic susceptibility is maximized at multiple-Q vectors related through lattice symmetry operations, exotic magnetic orderings such as vortex and skyrmion crystals may follow. Here, we show that a periodic array of nonmagnetic impurities, which can be realized through charge density wave ordering, leads to a rich phase diagram featuring a plethora of chiral magnetic phases, especially when there is a simple relation between the reciprocal vectors of the impurity superlattice and the magnetic Q vectors. We also investigate the effect of changing the impurity concentration or disturbing the impurity array with small quenched randomness. Lastly, alternative realizations of impurity superlattices are briefly discussed.
36 MATERIALS SCIENCE↗
Properties of rare-earth iron garnets from first principles
Here, structural and magnetic properties of rare-earth iron garnets (RIG), which contain 160 atoms per unit cell, are systematically investigated for rare-earth elements varying from La to Lu (and including Y), by performing spin polarized density-functional calculations. The effects of 4f electrons (as core or as valence electrons) on the lattice constant, internal coordinates, and bond lengths are found to be rather small, with these predicted structural properties agreeing rather well with available experiments. On the other hand, treating such electrons as valence electrons is essential to interpret the total magnetization measured in some RIG at low temperature, the different orientation and magnitude of the magnetizations that Fe and rare-earth ions can adopt and to also explain why some RIG have a compensation temperature while others do not. The magnetic exchange couplings and orbital-projected density of states are also reported for two representative materials, namely Gd 3 Fe 5 O 12 and Nd 3 Fe 5 O 12 , when accounting for their 4f electrons.
36 MATERIALS SCIENCE↗
Explicit inclusion of electronic correlation effects in molecular dynamics
Here, we design a quantum molecular dynamics method for strongly correlated electron metals. The strong electronic correlation effects are treated within a real-space version of the Gutzwiller variational approximation (GA), which is suitable for the inhomogeneity inherent in the process of quantum molecular dynamics (MD) simulations. We also propose an efficient algorithm based on the second-moment approximation to the electronic density of states for the search of the optimal variation parameters, from which the renormalized interatomic MD potentials are fully determined. By considering a minimal one-correlated-orbital Anderson model with parameterized spatial dependence of tight-binding hopping integrals, this fast GA-MD method is benchmarked with that using exact diagonalization to solve the GA variational parameters. The efficiency and accuracy are illustrated. We have demonstrated the effect of temperature coupled with electronic correlation on structural properties simulated with MD. This method will open up an unprecedented opportunity enabling large-scale quantum MD simulations of strongly correlated electronic materials.
36 MATERIALS SCIENCE↗