Topological quantum paramagnets are exotic states of matter, whose magnetic excitations have a topological band structure, while the ground state is topologically trivial. Here we show that a simple model of quantum spins on a honeycomb bilayer hosts a time-reversal-symmetry protected ${\mathbb{Z}}_2$ topological quantum paramagnet (topological triplon insulator) in the presence of spin-orbit coupling. The excitation spectrum of this quantum paramagnet consists of three triplon bands, two of which carry a nontrivial ${\mathbb{Z}}_2$ index. As a consequence, there appear two counterpropagating triplon excitation modes at the edge of the system. We compute the triplon edge state spectrum and the ${\mathbb{Z}}_2$ index for various parameter choices. We further show that upon making one of the Heisenberg couplings stronger, the system undergoes a topological quantum phase transition, where the ${\mathbb{Z}}_2$ index vanishes, to a different topological quantum paramagnet. In this case the counterpopagating triplon edge modes are disconnected from the bulk excitations and are protected by a chiral and a unitary symmetry. We discuss possible realizations of our model in real materials, in particular $d^4$ Mott insulators, and their potential applications.

We study the spread of a quantum-mechanical wave packet in a noisy environment, modeled using a tight-binding Hamiltonian. Despite the coherent dynamics, the fluctuating environment may give rise to diffusive behavior. When correlations between different level-crossing events can be neglected, we use the solution of the Landau-Zener problem to find how the diffusion constant depends on the noise. We also show that when an electric field or external disordered potential is applied to the system, the diffusion constant is suppressed with no drift term arising. The results are relevant to various quantum systems, including exciton diffusion in photosynthesis and electronic transport in solid-state physics.

Experimental studies demonstrate that the low-frequency dielectric permittivity of many magnetic materials is sensitive to the spin ordering, but the microscopic mechanisms of the observed phenomena still remain poorly understood. Here we report on the study of lattice dynamics and the spontaneous magnetodielectric effect using far-infrared and low-frequency dielectric spectroscopy in the model fluoroperovskites ${\mathrm{KCoF}}_3$ and ${\mathrm{RbCoF}}_3$ in the temperature range of 5–300 K, which includes the antiferromagnetic transition at $T_N=115$ K and 101 K, respectively. We show that the dielectric permittivity is mainly defined by the transverse TO and longitudinal LO infrared-active phonons and their specific contributions were determined. The anomalous growth of the low-frequency dielectric permittivity observed in ${\mathrm{KCoF}}_3$ with cooling is explained by the increase of the $\text{LO-TO}$ splitting of the lowest frequency phonon caused by about $7{\mathrm{cm}}^{-1}$ softening. An important conclusion is that, microscopically, the spontaneous magnetodielectric effect is caused by the frequency shifts of only those TO and LO phonons which change the ${180}^{\circ }$ angle of the superexchange ${\mathrm{Co}}^{2+}-{\mathrm{F}}^{1-}-{\mathrm{Co}}^{2+}$ pathway, thus resulting in its modulation due to the spin-phonon coupling. The observed anomalous softening of the lowest frequency phonon and increase of the low-frequency dielectric permittivity were interpreted as a manifestation of the ferroelectric instability in cubic fluoroperovskites.

We observe a hysteretic reorientation of the magnetic vortex lattice in the noncentrosymmetric superconductor ${\mathrm{Ru}}_7{\mathrm{B}}_3$, with the change in orientation driven by altering the magnetic field below $T_{\mathrm{c}}$. Normally a vortex lattice chooses either a single or degenerate set of orientations with respect to a crystal lattice at any given field or temperature, a behavior well described by prevailing phenomenological and microscopic theories. Here, in the absence of any typical VL structural transition, we observe a continuous rotation of the vortex lattice which exhibits a pronounced hysteresis and is driven by a change in magnetic field. We propose that this rotation is related to the spontaneous magnetic fields present in the superconducting phase, which are evidenced by the observation of time-reversal symmetry breaking, and the physics of broken inversion symmetry. Finally, we develop a model from the Ginzburg-Landau approach which shows that the coupling of these to the vortex lattice orientation can result in the rotation we observe.

We investigated the magnetic properties of the low-dimensional $\mathrm{Ba}{M}_2{\mathrm{Si}}_2{\mathrm{O}}_7$ ($M$ = Cu, Co, and Mn) system using both experimental measurements and theoretical calculations. Magnetization, specific heat, and single crystal neutron diffraction measurements have been performed on single crystal ${\mathrm{BaMn}}_2{\mathrm{Si}}_2{\mathrm{O}}_7$. The spin structure was determined and a magnetic phase diagram with applied field along the $b$ axis was constructed, which contains a spin flop transition around 6 T. Magnetization and specific heat measurements confirmed the presence of weak ferromagnetism in ${\mathrm{BaCo}}_2{\mathrm{Si}}_2{\mathrm{O}}_7$. Furthermore, we performed local-spin density approximation with on-site Coulomb energy ($\mathrm{LSDA}+U$) calculations for the $\mathrm{Ba}{M}_2{\mathrm{Si}}_2{\mathrm{O}}_7$ ($M$ = Cu, Co, and Mn) system. Based on the first-principles calculations, the origin of the magnetic differences of the three materials is discussed.

The ability of first-principles computational methods to reproduce ground-state crystal structure selection is key to their application in the discovery of new materials, and yet presents a formidable challenge due to the low-energy scale of the problem and lack of systematic error cancellation. The recently developed Strongly Constrained and Appropriately Normed (SCAN) functional is notable for accurately calculating physical properties such as formation energies and in particular, correctly predicting ground-state structures. Here, we attempt to rationalize the improved structure prediction accuracy in SCAN by investigating the relationship between preferred coordination environments, the description of attractive van der Waals (vdW) interactions, and the overall ground-state prediction in bulk main-group solids. We observe a systematic undercoordination error in the traditional Perdew, Burke, and Ernzerhof (PBE) functional which is not present in SCAN results and find that semiempirical dispersion corrections in the form of $\mathrm{PBE}+\mathrm{D}3$ fail to correct this error in a consistent or physical manner. We conclude that the medium-range vdW interaction is correctly parametrized in SCAN and yields meaningful relative energies between coordination environments.

We consider the fermionic $\mathrm{SU}(3)$ Hubbard model on the triangular lattice at $1/3$ filling in the presence of a three-sublattice staggered potential which provides the possibility to investigate the competition of charge and magnetic order in three-component systems. We show that, depending on the strength of the staggered potential $\mathrm{\Delta }$, the Hubbard interaction $U$ destabilizes the band insulator at small $U$ into the Mott insulator at large $U$ in three different ways with different intermediate phases. This leads to a rich phase diagram in the $U\text{−}\mathrm{\Delta }$ plane. Our results indicate that multicomponent systems show not only exotic states in the Mott regime as has been considered previously, but also interesting competition between charge and magnetic orders which can lead to the emergence of charge-ordered magnetic insulators and charge-ordered magnetic metals.

We demonstrate that single-layer graphene exhibits the electronic structure of a bilayer when it is connected to two gated bilayers. The valence and conduction band continua as well as the energy gap characteristic of gated bilayer are induced in the single layer; the gap persists for distances separating the bilayers up to 20 nm. Most importantly, we report on the appearance of a topologically protected gapless state localized in a single graphene layer when the distant bilayers differ in the stacking order. The one-dimensional current in the topological state flows in single-layer graphene, and this finding suggests a new exploitation of graphene in nanoelectronics.

A temperature driven structural phase transition between $\left(\sqrt{3}\times \sqrt{3}\right)$ and $\left(3\times 3\right)$ has been reported for Sn adsorbed on the Ge(111) surface, which does not occur for the analogous Sn/Si(111) system. This phase transition has been correlated to a softening of a low-frequency Sn vibration mode, referred to as dynamical fluctuation mode. We have determined the eigenfrequencies of the vibration modes of Sn/Ge(111) and Sn/Si(111) with high accuracy by in situ surface Raman spectroscopy in the temperature range between 300 and 40 K, and calculated the surface reconstructions and vibration eigenmodes by density functional theory. Our calculated vibration eigenfrequencies are in excellent agreement with the observed Raman peak positions and the calculated displacement patterns allow the assignment of all observed vibration modes. Our results for both adsorbate systems Sn/Ge(111) and Sn/Si(111) show the preservation of the global surface atom configuration over the whole investigated temperature range. The emergence of a backfolded Rayleigh wave at $\approx 50{\mathrm{cm}}^{-1}$ for Sn/Ge(111) below $\approx 200$ K is a clear signature of its transition to a static $\left(3\times 3\right)$ reconstruction. The gradual intensity increase of this mode upon further cooling suggests an order-disorder character of this transition.

Transport signatures of exchange gap opening because of magnetic proximity effect (MPE) are reported for bilayer structures of $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ thin films on yttrium iron garnet (YIG) and thulium iron garnet (TmIG) of perpendicular magnetic anisotropy (PMA). Pronounced negative magnetoresistance (MR) was detected, and attributed to an emergent weak localization (WL) effect superimposing on a weak antilocalization (WAL). Thickness-dependent study shows that the WL originates from the time-reversal-symmetry breaking of topological surface states by interfacial exchange coupling. The weight of WL declined when the interfacial magnetization was aligned toward the in-plane direction, which is understood as the effect of tuning the exchange gap size by varying the perpendicular magnetization component. Importantly, magnetotransport study revealed anomalous Hall effect (AHE) of square loops and anisotropic magnetoresistance (AMR) characteristic, typifying a ferromagnetic conductor in $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3/\mathrm{TmIG}$, and the presence of an interfacial ferromagnetism driven by MPE. Coexistence of MPE-induced ferromagnetism and the finite exchange gap provides an opportunity of realizing zero magnetic-field dissipation-less transport in topological insulator/ferromagnetic insulator heterostructures.

We compute the transport and chaos properties of lattices of quantum Sachdev-Ye-Kitaev islands coupled by single fermion hopping, and with the islands coupled to a large number of local, low energy phonons. We find two distinct regimes of linear-in-temperature $\left(T\right)$ resistivity, and describe the crossover between them. When the electron-phonon coupling is weak, we obtain the “incoherent metal” regime, where there is near-maximal chaos with front propagation at a butterfly velocity $v_B$, and the associated diffusivity $D_{\mathrm{chaos}}=v_B^2/\left(2\pi T\right)$ closely tracks the energy diffusivity. On the other hand, when the electron-phonon coupling is strong, and the linear resistivity is largely due to near-elastic scattering of electrons off nearly free phonons, we find that the chaos is far from maximal and spreads diffusively. We also describe the crossovers to low $T$ regimes where the electronic quasiparticles are well defined.

In a recent paper [Bardyn et al., Phys. Rev. X 8, 011035 (2018)], it was shown that the generalization of many-body polarization to mixed states can be used to construct a topological invariant that is also applicable to finite-temperature and nonequilibrium Gaussian states of lattice fermions. The many-body polarization defines an ensemble geometric phase that is identical to the Zak phase of a fictitious Hamiltonian, whose symmetries determine the topological classification. Here we show that in the case of Gaussian states of bosons, the corresponding topological invariant is always trivial. This also applies to finite-temperature states of bosons in lattices with a topologically nontrivial band structure. As a consequence, there is no quantized topological charge pumping for translationally invariant bulk states of noninteracting bosons.

The spin Hall torque induced by DC current was studied experimentally. A Pt/Co/Pt structure with perpendicular magnetic anisotropy was grown, and magnetization reversal due to the spin Hall effect was observed. An extra magnetic field perpendicular to the sample was applied in addition to a constant bias field. It was found that the switching current shifted in proportion to the additional perpendicular field. The spin Hall angle was estimated from this relation, and the small perpendicular magnetic field can be measured accurately based on this effect.

We show on a few examples of one-band materials with spheroidal Fermi surfaces and anisotropic order parameters that anisotropies ${\gamma }_H$ of the upper critical field and ${\gamma }_{\lambda }$ of the London penetration depth depend on temperature, a feature commonly attributed to multiband superconductors. The parameters ${\gamma }_H$ and ${\gamma }_{\lambda }$ may have opposite temperature dependences or may change in the same direction depending on the Fermi-surface shape and on the character of the gap nodes. For two-band systems, the behavior of anisotropies is affected by the ratios of bands densities of states, Fermi velocities, anisotropies, and order parameters. We investigate in detail the conditions determining the directions of temperature dependences of the two anisotropy factors.

We calculate the atomic and electronic structures of trirutile-type ${\mathrm{Li}}_x{\mathrm{FeF}}_3(x=0,0.25,0.5,0.75,\text{and}1)$ by first-principles calculations and evaluate the relative stability among the optimized structures by energy analysis. ${\mathrm{Li}}_{0.5}{\mathrm{FeF}}_3$ is more stable than the three-phase coexistence of ${\mathrm{FeF}}_3,{\mathrm{FeF}}_2$, and LiF, whereas the other compositions are unstable. The analyses of the local electron density, local atomic volume, and local atomic configurations show that the formal valence of Fe atoms decreases from trivalent (3+) to divalent (2+) after Li insertion. In addition, we calculate Fe $K$-edge x-ray absorption near-edge structure (XANES) spectra in ${\mathrm{Li}}_x{\mathrm{FeF}}_3$ and compare them with observed spectra. The calculated XANES spectra agree well with the corresponding observed spectra in areas such as the spectral shape and relative position of the main peaks associated with ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{2+}$. In particular, partial XANES spectra of ${\mathrm{Fe}}^{3+}$ in ${\mathrm{Li}}_x{\mathrm{FeF}}_3$, for $x=0.25,0.5$, and 0.75, have a specific peak between the main peaks, associated with ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{2+}$. The detailed study reveals that the energy level and intensity ratio of the ${\mathrm{Fe}}^{3+}$ main peaks depend on the adjacent cation site of Fe.