We use resonant soft x-ray diffraction to track the photoinduced dynamics of the antiferromagnetic structure in a $\mathrm{NdNi}{\mathrm{O}}_3$ thin film. Femtosecond laser pulses with a photon energy of 0.61 eV, resonant with electron transfer between long-bond and short-bond nickel sites, are used to excite the material and drive an ultrafast insulator-metal transition. Polarization-sensitive soft x-ray diffraction, resonant to the nickel $L_3$ edge, then probes the evolution of the underlying magnetic spiral as a function of time delay with 80 ps time resolution. By modeling the azimuthal dependence of the scattered intensity for different linear x-ray polarizations, we benchmark the changes of the local magnetic moments and the spin alignment. The measured changes are consistent with a reduction of the long-bond site magnetic moments and an alignment of the spins towards a more collinear structure at early time delays.

A key concept in the emerging field of spintronics is the electric field control of spin precession via the effective magnetic field generated by the Rashba spin orbit interaction (RSOI). Here we model a quantum wire as a system of electrons on a discrete one-dimensional chain with nearest neighbor hopping. By extensive density matrix renormalization group computations we demonstrate the presence of alternating spin current order in the gapped phases of this quantum wire with spatially modulated RSOI and repulsive electron-electron interactions. Our results are analytically supported by bosonization and by a mapping to a locally rotated spin basis.

We generalize the nonaffine theory of viscoelasticity for use with large, well-sampled systems of arbitrary chemical complexity. Having in mind predictions of mechanical and vibrational properties of amorphous systems with atomistic resolution, we propose an extension of the kernel polynomial method (KPM) for the computation of the vibrational density of states and the eigenmodes, including the $\mathrm{\Gamma }$ correlator of the affine force field, which is a key ingredient of lattice-dynamic calculations of viscoelasticity. We show that the results converge well to the solution obtained by direct diagonalization (DD) of the Hessian (dynamical) matrix. As is well known, the DD approach has prohibitively high computational requirements for systems with $N={10}^4$ atoms or larger. Instead, the KPM approach developed here allows one to scale up lattice dynamic calculations of real materials up to ${10}^6$ atoms, with a hugely more favorable (linear) scaling of computation time and memory consumption with $N$.

The delicate balance between spin-orbit coupling, Coulomb repulsion, and crystalline electric field interactions observed in Ir-based oxides is usually manifested as exotic magnetic behavior. Here we investigate the evolution of the exchange coupling between Co and Ir for partial La substitution by Ca in ${\mathrm{La}}_2{\mathrm{CoIrO}}_6$. A great advantage of the use of ${\mathrm{Ca}}^{2+}$ as a replacement for ${\mathrm{La}}^{3+}$ is the similarity of their ionic radii. Thus, the observed magnetic changes can more easily be associated with electronic variations. A thorough investigation of the structural, electronic, and magnetic properties of the ${\mathrm{La}}_{2-x}{\mathrm{Ca}}_x{\mathrm{CoIrO}}_6$ system was carried out by means of synchrotron x-ray powder diffraction, muon spin rotation and relaxation $\left(\mu \mathrm{SR}\right)$, AC and DC magnetization, x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism, Raman spectroscopy, electrical resistivity, and dielectric permittivity. Our XAS results show that up to 25% Ca substitution at the La site results in the emergence of ${\mathrm{Co}}^{3+}$, possibly in a high-spin state, while the introduction of a larger amount of Ca leads to an increase in the Ir valence. The competing magnetic interactions resulting from the mixed valences lead to the coexistence of a magnetically ordered and an emerging spin-glass (SG) state for the doped samples. Our $\mu \mathrm{SR}$ results indicate that for ${\mathrm{La}}_2{\mathrm{CoIrO}}_6$ a nearly constant fraction of a paramagnetic (PM) phase persists down to low temperatures, possibly related to the presence of a small amount of ${\mathrm{Ir}}^{3+}$ and to the antisite disorder at Co/Ir sites. For doped compounds the PM phase freezes below 30 K, but there is still some dynamics associated with the SG. The dielectric data obtained for the parent compound and the one with 25% Ca doping indicate a possible magnetodielectric effect, which is discussed in terms of the electron hopping between the transition-metal ions, the antisite disorder at Co/Ir sites, and the distorted crystalline structure.

We analyze the low-energy dynamics of quasi-one-dimensional, large-$S$ quantum antiferromagnets with easy-axis anisotropy, using a semiclassical nonlinear sigma model. The saddle point approximation leads to a sine-Gordon equation which supports soliton solutions. These correspond to the movement of spatially extended domain walls. Long-range magnetic order is a consequence of a weak interchain coupling. Below the ordering temperature, the coupling to nearby chains leads to an energy cost associated with the separation of two domain walls. From the kink-antikink two-soliton solution, we compute the effective confinement potential. At distances large compared to the size of the solitons the potential is linear, as expected for pointlike domain walls. At small distances the gradual annihilation of the solitons weakens the effective attraction and renders the potential quadratic. From numerically solving the effective one-dimensional Schrödinger equation with this nonlinear confinement potential we compute the soliton bound state spectrum. We apply the theory to ${\mathrm{CaFe}}_2{\mathrm{O}}_4$, an anisotropic $S=5/2$ magnet based upon antiferromagnetic zigzag chains. Using inelastic neutron scattering, we are able to resolve seven discrete energy levels for spectra recorded slightly below the Néel temperature $T_{\text{N}}\approx 200$ K. These modes are well described by our nonlinear confinement model in the regime of large spatially extended solitons.

The dynamic magnetic properties of the Kondo-insulator state in $\mathrm{Ce}{\mathrm{Fe}}_2{\mathrm{Al}}_{10}$ (spin gap, resonance mode) have been investigated using polarized neutrons on a single crystal of pure $\mathrm{Ce}{\mathrm{Fe}}_2{\mathrm{Al}}_{10}$. The results indicate that the magnetic excitations are polarized mainly along the orthorhombic $a$ axis and their dispersion along the orthorhombic $c$ direction could be determined. Polycrystalline samples of Nd- and Rh-doped $\mathrm{Ce}{\mathrm{Fe}}_2{\mathrm{Al}}_{10}$ were also studied by the time-of-flight technique, with the aim of finding out how the low-energy magnetic excitation spectra change upon isoelectronic substitution of a rare-earth element (Nd) on the magnetic Ce site or electron doping (Rh) on the transition-element Fe sublattice. The introduction of magnetic Nd impurities strongly modifies the spin gap in the Ce dynamic magnetic response and causes the appearance of a quasielastic signal. The crystal-field excitations of Nd, studied in both $\mathrm{La}{\mathrm{Fe}}_2{\mathrm{Al}}_{10}$ and $\mathrm{Ce}{\mathrm{Fe}}_2{\mathrm{Al}}_{10}$, also reveal a significant influence of $f$-electron hybridization (largest in the case of Ce) on the crystal-field potential. As a function of the Rh concentration, a gradual change is observed from a Kondo-insulator to a metallic Kondo-lattice response, likely reflecting the decrease in the hybridization energy.

The quest for new realizations of higher-order topological system has garnered much recent attention. In this work, we propose a paradigmatic brick lattice model where corner modes require protection by nonsymmorphic symmetry in addition to two commuting mirror symmetries. Unlike the well-known square corner mode lattice, it has an odd number of occupied bands, which necessitates a different definition for the ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ topological invariant. By studying both the quadrupolar polarization and effective edge model, our study culminates in a phase diagram containing two distinct topological regimes. Our brick lattice corner modes can be realized in an $RLC$ circuit setup and detected via colossal “topolectrical” resonances.

We report a combined experimental and theoretical study of the surface and bulk electronic structure of aluminium diboride, a nonsuperconducting sister compound of the superconductor ${\mathrm{MgB}}_2$. We perform angle-resolved photoemission measurements with variable photon energy, and compare them to density functional theory calculations to disentangle the surface and bulk contributions to the measured spectra. Aluminium diboride is known to be aluminium deficient, ${\mathrm{Al}}_{1-\delta }{\mathrm{B}}_2$, which would be expected to lead to a hole doping as compared to the nominally stoichimoetric compound. Nonetheless, we find that the bulk $\sigma$ states, which mediate superconductivity in ${\mathrm{MgB}}_2$, remain more than $600\mathrm{meV}$ below the Fermi level. However, we also observe $\sigma$ states originating from the boron terminated surface, with an order of magnitude smaller binding energy of $70\mathrm{meV}$, and demonstrate how surface hole-doping can bring these across the Fermi level.

Magnetic topological insulators (TIs) with nontrivial topological electronic structure and broken time-reversal symmetry exhibit various exotic topological quantum phenomena. The realization of such exotic phenomena at high temperature is one of the central topics in this area. We reveal that ${\mathrm{MnBi}}_6{\mathrm{Te}}_{10}$ is a magnetic TI with an antiferromagnetic ground state below 10.8 K whose nontrivial topology is manifested by Dirac-like surface states. The ferromagnetic axion insulator state with $Z_4=2$ emerges once spins are polarized at a field as low as 0.1 T, accompanied with saturated anomalous Hall resistivity up to 10 K. Such a ferromagnetic state is preserved even with an external field down to zero at 2 K. Theoretical calculations indicate that the few-layer ferromagnetic ${\mathrm{MnBi}}_6{\mathrm{Te}}_{10}$ is also topologically nontrivial with a nonzero Chern number. Angle-resolved photoemission spectroscopy experiments further reveal three types of Dirac surface states arising from different terminations on the cleavage surfaces, one of which has insulating behavior with an energy gap of ∼28 meV at the Dirac point. These outstanding features suggest that ${\mathrm{MnBi}}_6{\mathrm{Te}}_{10}$ is a promising system to realize various topological quantum effects at zero field and high temperature.

A two-dimensional (2D) topological band is characterized by the (first) Chern number. The zero and nonzero Chern numbers usually represent the trivial and nontrivial band topologies, respectively. In this paper, we study an extended Qi-Wu-Zhang model that hosts the topological band with a zero Chern number. We show that the zero Chern number band is topologically nontrivial, characterized by the half-integer wave polarization. The nontrivial topology is manifested by the anisotropic in-gap edge states, which are verified to be robust against 2D particle-hole symmetric disorders.

We study by a consistent mean-field scheme the role on the single- and two-particle properties of a local electron-electron repulsion in the Bernevig, Hughes, and Zhang model of a quantum spin Hall insulator. We find that the interaction fosters the intrusion between the topological and nontopological insulators of an insulating and magnetoelectric phase that breaks spontaneously inversion and time-reversal symmetries but not their product. The approach to this phase from both topological and nontopological sides is signaled by the softening of two exciton branches, i.e., whose binding energy reaches the gap value, that possess, in most cases, finite and opposite Chern numbers, thus allowing this phase to be regarded as a condensate of topological excitons. We also discuss how those excitons, and especially their surface counterparts, may influence the physical observables.

We investigated the electronic structure of the $\mathrm{Si}\left(111\right)–7\times 7$ surface below 20 K by scanning tunneling and photoemission spectroscopies and by density functional theory calculations. Previous experimental studies have questioned the ground state of this surface, which is expected to be metallic in a band picture because of the odd number of electrons per unit cell. Our differential conductance spectra instead show the opening of an energy gap at the Fermi level and a significant temperature dependence of the electronic properties, especially for the adatoms at the center of the unfaulted half of the unit cell. Complementary photoemission spectra with improved correction of the surface photovoltage shift corroborate the differential conductance data and demonstrate the absence of surface bands crossing the Fermi level at 17 K. These consistent experimental observations point to an insulating ground state and contradict the prediction of a metallic surface obtained by density functional theory in the generalized gradient approximation. The calculations indicate that this surface has or is near a magnetic instability, but remains metallic in the magnetic phases even including correlation effects at mean-field level. We discuss possible origins of the observed discrepancies between experiments and calculations.

Using the tight-binding approximation combined with the mean-field Hubbard model and a Green's function approach, we designed a spin thermoelectric device in which the electrodes and the channel region consist of zigzag phosphorene nanoribbons. Our theoretical results show that applying a gate voltage only in the channel region induces a spin-semiconducting behavior with spin-polarized midgap states. Furthermore, the appearance of the localized and asymmetric electron-hole transmission coefficients induces a giant spin Seebeck effect with a spin-dependent current by applying a thermal gradient. Additionally, the spin-Seebeck values for zigzag phosphorene nanoribbons could be larger than those calculated for magnetized zigzag graphene nanoribbons. More interestingly, the values and the direction of the spin Seebeck effect as well as the spin-dependent currents can be easily manipulated by controlling the applied gate voltage or the magnetic state of the channel at room temperature.

The spin-selective transport through helical molecules has been a hot topic in condensed matter physics, because it develops a new research direction in spintronics, i.e., chiro-spintronics. Double-stranded DNA (dsDNA) molecules have been considered as promising candidates to study this topic, since the chiral-induced spin selectivity (CISS) effect in dsDNA was observed in experiment. Considering that the dsDNA molecules are usually flexible in mechanical properties, vibration may be one of important factors to influence the CISS effect. Here, we investigate the influences of electron-vibration interaction (EVI) on the spin-selective transport in dsDNA molecules. We uncover that the EVI not only enhances the CISS effect and the spin polarization ($P_s$) in dsDNA, but also induces a series of new spin-splitting transmission modes. More interesting, these vibration-induced transmission spectra tend to host the same $P_s$ values as those of the original spin-splitting transmission modes, making the $P_s$ spectra to display as a continuous platform even in the energy gap. Our work not only provides us a deep understanding into the influence of vibrations on the CISS effect in helical molecules, but also puts forth a feasible route to detect the vibration-induced spin-polarized transport in low-dimensional molecular systems.

At temperatures higher than 341 K, vanadium dioxide (${\mathrm{VO}}_2$) is a strongly correlated metal with resistivity exceeding the Mott-Ioffe-Regel limit. Its electronic thermal conductivity is lower than that predicted by the Wiedemann-Franz (WF) law, and can be explained by nonquasiparticle transport where heat and charge currents follow separate diffusive modes. In contradiction, the Wiedemann-Franz law is a direct consequence of quasiparticle transport where charge carriers are elastically scattered. In this work, we enhance elastic electron scattering in ${\mathrm{VO}}_2$ by introducing atomic disorder with ion irradiation. A gradual and eventually full recovery of the WF law is observed at high defect densities. This observation provides an example that connects hydrodynamic quasiparticle transport to nonquasiparticle transport in metallic systems.

We consider weak topological insulators with a twofold rotation symmetry around their “dark” direction and show that these systems can be endowed with the topological crystalline structure of a higher-order topological insulator protected by rotation symmetry. These hybrid-order weak topological insulators display surface Dirac cones on all surfaces. Translational symmetry breaking perturbations gap the Dirac cones on the side surfaces leaving anomalous helical hinge modes behind. We also prove that the existence of this topological phase comes about due to a novel crystalline topological invariant of quantum spin-Hall insulators that can neither be revealed by symmetry indicators nor using Wilson loop invariants. Considering the minimal symmetry requirements, we anticipate that our findings could apply to a large number of weak topological insulators.

In two-dimensional electron systems (2DES), a hydrodynamic regime with remarkable viscous effects is recognized when the sample width is small enough and momentum-conserving scattering is dominant. Based on hydrodynamic calculations for a 2DES, we obtain large effective viscosity coefficients, which lead to large negative magnetoresistance (MR) and Hall resistance due to the effect of an in-plane Stern-Gerlach force on the viscosity components. The spin-dependent viscous effect manifests itself via both longitudinal and Hall resistances. The negative MR and Hall resistance are markedly sensitive to the perpendicular magnetic field with amplitude around zero, which would be useful in MR sensors and magnetic recording devices.

We analyze the quasienergy spectrum, the valence to conduction-band transition probabilities, and the photo-induced density currents of a tilted anisotropic Dirac material subject to linearly and circularly polarized electromagnetic fields. The quasienergy spectrum is numerically calculated from the monodromy matrix of the Schrödinger equation via the Floquet theorem for arbitrarily intense electromagnetic fields. The monodromy matrix method developed here is much more efficient for obtaining the evolution operator and Floquet spectrum of time-driven systems than the traditional diagonalization using replicas, as this last method requires truncation in both the number of replicas and system size. To assess the valence to conduction-band transition times, we deduced a Rabi-like formula in the rotating wave approximation. In the strong-field regime, the spectrum as a function of the momentum components divides into two very distinctive regions. In the first, located around the Dirac point, the quasi spectrum is significantly distorted by the field as the electronic parameters are renormalized by electronic dressing. In the second, all the characteristics of the free carrier spectrum are retained. Linearly polarized light anisotropically deforms the spectrum according to the field polarization direction. Dirac-like points form around the original Dirac point. The quasi spectrum of circularly polarized light, instead, exhibits a gap formation in the Dirac point and has elliptical symmetry. We show that, in contrast to the single-photon resonant transitions that characterize the weak-field regime, the strong-field regime is dominated by multiphoton resonances. These processes also manifest themselves in the generation of high harmonics in the density current.

Semilocal exchange functionals are propitious in describing several properties of solids due to computational competence. For calculating band gaps of solids, the underestimation of generalized gradient approximation (GGA) functionals is known. The next rung of GGA in Jacob's ladder, i.e., meta-GGA, is expected to improve the band-gap calculation. In this regard, exchange-only potentials are very effective in producing accurate band gaps of solids. But, due to the presence of lattice-dependent parameters, these methods are incompetent for monolayers or slabs. Here, we show the potency of advance meta-GGA energy functionals in elucidating the band gaps of different layered structures. The recently proposed MGGAC meta-GGA functional by Patra et al. Phys. Rev. B 100, 155140 (2019) has proved to be very promising for band gaps within the semilocal treatment of density-functional theory. This conclusion is acquired by applying these functionals to encouraging examples of doped graphene, graphane, and halogenated graphene having insulating band gaps. Also, the improved values of energies of hydrogen-terminated Si(111) surface states are demonstrated with this meta-GGA functional.