We present a quantitative and comparative study of magnetic-field-driven domain-wall depinning transition in different ferromagnetic ultrathin films over a wide range of temperature. We reveal a universal scaling function accounting for both drive and thermal effects on the depinning transition, including critical exponents. The consistent description we obtain for both the depinning and subthreshold thermally activated creep motion should shed light on the universal glassy dynamics of thermally fluctuating elastic objects pinned by disordered energy landscapes.

The spin-order-induced ferroelectric antiferromagnet $\mathrm{N}{\mathrm{i}}_3\mathrm{Te}{\mathrm{O}}_6$ transcends the magnetoelectric performance of all other single-phase multiferroics because it exhibits nonhysteretic colossal magnetoelectric coupling [Y. S. Oh, S. Artyukhin, J. J. Yang, V. Zapf, J. W. Kim, D. Vanderbilt, and S.-W. Cheong, Nat Commun. 5, 3201 (2014)]. We investigated spin and lattice excitations in $\mathrm{N}{\mathrm{i}}_3\mathrm{Te}{\mathrm{O}}_6$ by a combination of infrared, Raman, and THz spectroscopies. Two spin excitations (near 13 and $35\mathrm{c}{\mathrm{m}}^{-1}$) were observed simultaneously in Raman and time-domain THz spectra below the Néel temperature $T_{\mathrm{N}}=53\mathrm{K}$. We propose to assign them to electromagnons, which are activated by the dynamic magnetoelectric coupling. A third magnon is seen only in the Raman spectra near $206\mathrm{c}{\mathrm{m}}^{-1}$.

The frustrated magnet ${\mathrm{SrDy}}_2{\mathrm{O}}_4$ exhibits a field-induced phase with a magnetization plateau at $1/3$ of the saturation value for magnetic fields applied along the $b$ axis. We report here a neutron scattering study of the nature and symmetry of the magnetic order in this field-induced phase. Below $T\approx 0.5$ K, there are strong hysteretic effects, and the order is short- or long-ranged for zero-field and field cooling, respectively. We find that the long-range ordered magnetic structure within the zigzag chains is identical to that expected for the one-dimensional axial next-nearest neighbor Ising (ANNNI) model in longitudinal fields. The long-range ordered structure in field contrasts with the short-range order found at zero field, and is probably reached through enhanced quantum fluctuations with increasing fields.

The spin Peltier effect (SPE), heat-current generation as a result of spin-current injection, has been investigated in alternately stacked Pt/${\mathrm{Fe}}_3{\mathrm{O}}_4$ multilayer films. The temperature modulation induced by the SPE in the $[\text{Pt}/{\mathrm{Fe}}_3{\mathrm{O}}_4]\times n$ films was found to be significantly enhanced with increasing the number of Pt/${\mathrm{Fe}}_3{\mathrm{O}}_4$ bilayers $n$. This SPE enhancement is much greater than that expected for a simple stack of independent Pt/${\mathrm{Fe}}_3{\mathrm{O}}_4$ bilayers. The observed $n$ dependence of the SPE can be explained by introducing spin-current redistribution in the multilayer films in the thickness direction, in a manner similar to the enhancement of the spin Seebeck effect in multilayers.

We have developed a generalized electronic phase separation model of high-temperature cuprate superconductors that links the two distinct energy scales of the superconducting (SC) and pseudogap (PG) phases via a charge-density-wave (CDW) state. We show that simulated electronic-density modulations resembling the charge order modulations detected in cuprates intertwine the SC and charge orders by localizing charge and providing the energy scale for a spatially periodic SC attractive potential. Bulk superconductivity is achieved with the inclusion of Josephson coupling between nanoscale domains of intertwined fluctuating CDW and SC orders, and local SC phase fluctuations give rise to the Fermi arcs along the nodal directions of the SC gap. We demonstrate the validity of the model by reproducing the hole-doping dependence of the PG onset temperature $T^{*}$, and the SC transition temperature $T_c$ of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{\mathrm{y}}$ and ${\mathrm{Bi}}_{2-\mathrm{y}}{\mathrm{Pb}}_{\mathrm{y}}{\mathrm{Sr}}_{2-\mathrm{z}}{\mathrm{La}}_{\mathrm{z}}{\mathrm{CuO}}_{6+\delta }$. The results show that the periodicity of the CDW order is controlled by the PG energy scale, and the hole-doping dependence of the SC energy gap is controlled by the charge ordering.

Oxygen-deficient ${\mathrm{TiO}}_2$ in the rutile structure as well as the ${\mathrm{Ti}}_3{\mathrm{O}}_5$ Magnéli phase is investigated within the charge self-consistent combination of density functional theory with dynamical mean-field theory. It is shown that an isolated oxygen vacancy (${\mathrm{V}}_{\mathrm{O}}$) in titanium dioxide is not sufficient to metallize the system at low temperatures. In a semiconducting phase, an in-gap state is identified at ${\varepsilon }_{\mathrm{IG}}^{}\sim -0.75\mathrm{eV}$ in excellent agreement with experimental data. Bandlike impurity levels, resulting from a threefold ${\mathrm{V}}_{\mathrm{O}}$-Ti coordination as well as entangled $(t_{2g},e_g)$ states, become localized due to site-dependent electronic correlations. Charge localization and strong orbital polarization occur in the ${\mathrm{V}}_{\mathrm{O}}$-near Ti ions, the details of which can be modified by a variation of the correlated subspace. At higher oxygen vacancy concentration, a correlated metal is stabilized in the Magnéli phase. A ${\mathrm{V}}_{\mathrm{O}}$-defect rutile structure of identical stoichiometry shows key differences in the orbital-resolved character and the spectral properties. Charge disproportionation is vital in the oxygen-deficient compounds, but obvious metal-insulator transitions driven or sustained by charge order are not identified.

We consider the Kitaev chain model with finite and infinite range in the hopping and pairing parameters, looking in particular at the appearance of Majorana zero-energy modes and massive edge modes. We study the system both in the presence and in the absence of time-reversal symmetry, by means of topological invariants and exact diagonalization, disclosing very rich phase diagrams. In particular, for extended hopping and pairing terms, we can get as many Majorana modes at each end of the chain as the neighbors involved in the couplings. Finally, we generalize the transfer matrix approach useful to calculate the zero-energy Majorana states at the edges for a generic number of coupled neighbors.

We study the scaling of the (basis-dependent) Shannon entropy for two-dimensional quantum antiferromagnets with Néel long-range order. We use a massless free-field description of the gapless spin wave modes and phase space arguments to treat the fact that the finite-size ground state is rotationally symmetric, while there are degenerate physical ground states which break the symmetry. Our results show that the Shannon entropy (and its Rényi generalizations) possesses some universal logarithmic term proportional to the number $N_{\text{NG}}$ of Nambu-Goldstone modes. In the case of a torus, we show that $S_{n>1}\simeq \mathrm{const}.\times N+\frac{N_{\text{NG}}}{4}\frac{n}{n-1}lnN$ and $S_1\simeq \mathrm{const}.\times N-\frac{N_{\text{NG}}}{4}lnN$, where $N$ is the total number of sites and $n$ the Rényi index. The result for $n>1$ is in reasonable agreement with the quantum Monte Carlo results of Luitz et al. [Phys. Rev. Lett. 112, 057203 (2014)], and qualitatively similar to those obtained previously for the entanglement entropy. The Shannon entropy of a line subsystem (embedded in the two-dimensional system) is also considered. Finally, we present some density-matrix renormalization group (DMRG) calculations for a spin-$\frac{1}{2}$ XY model on the square lattice in a cylinder geometry. These numerical data confirm our findings for logarithmic terms in the $n=\infty$ Rényi entropy (also called $-ln{p}_{\max })$. They also reveal some universal dependence on the cylinder aspect ratio, in good agreement with the fact that, in that case, $p_{\max }$ is related to a noncompact free-boson partition function in dimension 1+1.

Using the nonequilibrium Keldysh formalism, we investigate the spatial relation between the electrochemical potential measured in scanning tunneling potentiometry, and local current patterns over the entire range from the quantum to the classical transport regime. These quantities show similar spatial patterns near the quantum limit but are related by Ohm's law only in the classical regime. We demonstrate that defects induce a Landauer residual resistivity dipole in the electrochemical potential with the concomitant spatial current pattern representing the field lines of the dipole.

In this work, we use density functional theory to predict the optical properties of ${\mathrm{Ti}}_x{\mathrm{Si}}_{1-x}{\mathrm{O}}_2$ solid solutions. The special quasirandom structure method and the simulated annealing procedure were applied to produce models of crystalline and amorphous ${\mathrm{Ti}}_x{\mathrm{Si}}_{1-x}{\mathrm{O}}_2$. These were fully structurally optimized by using the vasp package, while their electronic structure and optical properties were subsequently calculated by using the wien2k package employing the TB-mBJ potential. The calculated band gaps for a-${\mathrm{Ti}}_x{\mathrm{Si}}_{1-x}{\mathrm{O}}_2$ evaluated by using the Tauc-like fitting approach are 8.53 eV for ${\mathrm{SiO}}_2$, quickly decreasing to 4.0 eV at $x=0.19$, 3.52 eV at $x=0.34$, and 3.24 eV for ${\mathrm{TiO}}_2$. Experimental samples were prepared by means of plasma-enhanced chemical vapor deposition to support the calculations. Ellipsometry and spectrophotometry yield a compositional trend for the experimental optical band gap comparable with our predictions: a quick decrease from 7.94 eV for pure ${\mathrm{SiO}}_2$ to 3.91 eV at $x=0.15$, followed by a much slower decrease over the rest of the composition range ending at 3.26 eV for pure ${\mathrm{TiO}}_2$. A detailed analysis of anatase and rutile-based solid solutions reveals the introduction of silicon-induced oxygen states into the band gap in the ${\mathrm{TiO}}_2$-rich composition region, which results in the predicted reduction of the band gap. However, we show that the optical absorption of those states is negligible. We have obtained good agreement between the calculated and measured imaginary part of the dielectric function ${\varepsilon }_i$, especially for the ${\mathrm{TiO}}_2$-rich compositions. Finally, we predict an almost-linear refractive index change at 632.8 nm between a-${\mathrm{SiO}}_2$ (1.36) and a-${\mathrm{TiO}}_2$ (2.34), which was experimentally confirmed.

The recent experimental discovery of three-dimensional (3D) materials hosting a strong Rashba spin-orbit coupling calls for the theoretical investigation of their transport properties. Here we study the zero-temperature dc conductivity of a 3D Rashba metal in the presence of static diluted impurities. We show that, at variance with the two-dimensional case, in 3D systems, spin-orbit coupling affects dc charge transport in all density regimes. We find in particular that the effect of spin-orbit interaction strongly depends on the direction of the current, and we show that this yields strongly anisotropic transport characteristics. In the dominant spin-orbit coupling regime where only the lowest band is occupied, the conductivity anisotropy is governed entirely by the anomalous component of the renormalized current. We propose that measurements of the conductivity anisotropy in bulk Rashba metals may give a direct experimental assessment of the spin-orbit strength.

We report the magnetoresistance (MR), Hall effect, and de Haas–van Alphen (dHvA) effect studies of single crystals of tungsten carbide, WC, which is predicted to be a new type of topological semimetal with triply degenerate nodes. With the magnetic field rotated in the plane perpendicular to the current, WC shows a field induced metal-to-insulator-like transition and large nonsaturating quadratic MR at low temperatures. As the magnetic field parallel to the current, a pronounced negative longitudinal MR only can be observed for a certain direction of current flow. The Hall effect indicates WC is a perfect compensated semimetal, which may be related to the large nonsaturating quadratic MR. The analysis of dHvA oscillations reveals that WC is a multiband system with small cross-sectional areas of Fermi surface and light cyclotron effective masses. Our results indicate that WC is an ideal platform to study the recently proposed “new fermions” with triply degenerate crossing points.

The ${\left[{\mathrm{Bi}}_2{M}_2{\mathrm{O}}_4\right]}_p{\mathrm{CoO}}_2$ materials ($M=\text{Ca}$, Sr, and Ba) were studied to clarify the effect of the lattice incommensurability on electronic properties using angle-resolved photoemission spectroscopy and transmission electron microscopy (TEM). Results show that the insulating behavior is characterized by a spectral weight for binding energies higher than 2.0 eV. Moreover, the spectral shape is modified as a function of the incident photon energy, demonstrating a close relationship between the electrical properties and interlayer coupling. TEM results show that the effect of the lattice mismatch differs for different misfit parameters $p$. We therefore conclude that the carrier concentration and the chemical environment at the misfit interface, which depend on the degree of incommensurability, mutually determine the electronic properties of the system.

Disordered quantum antiferromagnets in two-dimensional compounds have been a focus of interest in the last years due to their exotic properties. However, with very few exceptions, the ground states of the corresponding Hamiltonians are notoriously difficult to simulate making their characterization and detection very elusive, both theoretically and experimentally. Here we propose a method to signal quantum disordered antiferromagnets by doing exact diagonalization in small lattices using random boundary conditions and averaging the observables of interest over the different disorder realizations. We apply our method to study the Heisenberg spin-1/2 model in an anisotropic triangular lattice. In this model, the competition between frustration and quantum fluctuations might lead to some spin-liquid phases as predicted from different methods ranging from spin-wave mean-field theory to 2D-DMRG or PEPS. Our method accurately reproduces the ordered phases expected of the model and signals quantum disordered phases by the presence of a large number of quasidegenerate ground states together with an undefined local order parameter. The method presents a weak dependence on finite-size effects.

We investigate the thermoelectric current and heat conductance in a chiral Josephson contact on a surface of a three-dimensional topological insulator, covered with superconducting and magnetic insulator films. The contact consists of two junctions of Majorana and Dirac channels next to two superconductors. Geometric asymmetry results in a supercurrent without a phase bias. The interference of Dirac fermions causes oscillations of the electric and heat currents with an unconventional period $2{\mathrm{\Phi }}_0=h/e$ as functions of the Aharonov-Bohm flux. Due to the gapless character of Majorana modes, there is no threshold for the thermoelectric effect, and the current-flux relationship is nonsinusoidal. Depending on the magnetic flux, the direction of the electric current can be both from the hot to the cold lead and vice versa.

Graphene, a very intriguing two-dimensional Dirac electronic system with high carrier mobility, is promising for spintronics. However, the long-range ferromagnetic order is always absent in pristine graphene. Here we report the fabrication and transport properties of graphene-$\mathrm{BiFe}{\mathrm{O}}_3$ heterostructures. It is found that the magnetic proximity effect results in a strong Zeeman splitting in graphene with the exchange field up to hundreds of tesla. The $\nu =0$ quantum Hall state of graphene is further transformed into a ferromagnetic state or a canted antiferromagnetic state in the presence of a perpendicular magnetic field. Our findings in graphene/$\mathrm{BiFe}{\mathrm{O}}_3$ heterostructure are therefore promising for future spintronics.

Atomically thin semiconducting transition-metal dichalcogenides enable new insight into physics of many-body effects mediated by Coulomb and electron-phonon interactions. We report photoluminescence (PL) and reflectivity contrast (RC) measurements of excitons ($X$) and trions ($T$) and Raman spectra of phonons in monolayers (MLs) of $\mathrm{Mo}{\left({\mathrm{S}}_y{\mathrm{Se}}_{1-y}\right)}_2$ alloys with sulfur mole content up to $y=0.5$. We find the phonon energy crossing the trion binding energy with the sulfur mole content. Binary ${\mathrm{MoSe}}_2$ and ternary $\mathrm{Mo}{\left({\mathrm{S}}_y{\mathrm{Se}}_{1-y}\right)}_2$ alloys exhibit contrasting behavior in the temperature evolution of excitons and trions PL intensity from $T=7$ to 295 K. In ${\mathrm{MoSe}}_2$, the trion dominates PL spectra at low temperatures but the exciton dominates PL at higher temperature. In contrast, in ternary $\mathrm{Mo}{\left({\mathrm{S}}_y{\mathrm{Se}}_{1-y}\right)}_2$ alloys trions dominate PL spectra at all measured temperatures, with the trion to exciton PL intensity ratio increasing with sulfur content. We attribute the strong increase of the trion PL intensity in $\mathrm{Mo}{\left({\mathrm{S}}_y{\mathrm{Se}}_{1-y}\right)}_2$ MLs with increase of sulfur mole content to two effects: (i) strong increase of exciton-trion coupling mediated by the optical phonon, which is realized by tuning phonon energy through trion binding energy, and (ii) significant increase of two-dimensional electron gas concentration. We also demonstrate that increasing sulfur content in $\mathrm{Mo}{\left({\mathrm{S}}_y{\mathrm{Se}}_{1-y}\right)}_2$ alloys increases total PL intensity at high temperature. With the temperature growth from 7 to 295 K, the total PL in ${\mathrm{MoSe}}_2$ decreases about three orders of magnitude more than in $\mathrm{Mo}{\left({\mathrm{S}}_{0.5}{\mathrm{Se}}_{0.5}\right)}_2$.

We show via ab initio calculations and an electrostatic model that the notoriously low, but positive, electron affinity of bulk diamond becomes negative for hydrogen passivated nanodiamonds and argue that this peculiar situation (type-II offset with a vacuum level at nearly midgap) and the three further conditions: (i) a surface dipole with positive charge on the outside layer, (ii) a spherical symmetry, and (iii) a dielectric mismatch at the surface, results in the emergence of a peculiar type of surface state localized just outside the nanodiamond. These states are referred to as “surface-bound states” and have consequently a strong environmental sensitivity. These type of states should exist in any nanostructure with negative electron affinity. We further quantify the band offsets of different type of nanostructures as well as the exciton binding energy and contrast the results with results for “conventional” silicon quantum dots.

We investigate theoretically the dynamics of a Josephson junction in the framework of the resistively shunted junction model. We consider a junction that hosts two supercurrent contributions: a $2\pi$ and a $4\pi$ periodic in phase, with intensities $I_{2\pi }$ and $I_{4\pi }$, respectively. We study the size of the Shapiro steps as a function of the ratio of the intensity of the mentioned contributions, i.e., $I_{4\pi }/I_{2\pi }$. We provide detailed explanations where to expect clear signatures of the presence of the $4\pi$-periodic contribution as a function of the external parameters: the intensity ac bias $I_{\text{ac}}$ and frequency ${\omega }_{\text{ac}}$. On the one hand, in the low ac-intensity regime (where $I_{\text{ac}}$ is much smaller than the critical current $I_{\text{c}}$), we find that the nonlinear dynamics of the junction allows the observation of only even Shapiro steps even in the unfavorable situation where $I_{4\pi }/I_{2\pi }\ll 1$. On the other hand, in the opposite limit ($I_{\text{ac}}\gg I_{\text{c}}$), even and odd Shapiro steps are present. Nevertheless, even in this regime, we find signatures of the $4\pi$ supercurrent in the beating pattern of the even step sizes as a function of $I_{\text{ac}}$.

We investigate the nonequilibrium transport properties of a disordered molecular nanowire. The nanowire is regarded as a quasi-one-dimensional organic crystal composed of self-assembled molecules. One orbital and a single random energy are assigned to each molecule while the intermolecular coupling does not fluctuate. Consequently, electronic states are expected to be spatially localized. We consider the regime of strong localization, namely, the localization length is smaller than the length of the molecular wire. Electron-vibron interaction, taking place at each single molecule, is also considered. We investigate the interplay between static disorder and electron-vibron interaction in response to either an applied electric bias or a temperature gradient. To this end, we calculate the electric and heat currents when the nanowire is connected to leads, using the Keldysh nonequilibrium Green's function formalism. At intermediate temperature, scattering by disorder dominates both charge and heat transport. We find that the electron-vibron interaction enhances the effect of the disorder on the transport properties due to the decrease of the coherent electron tunneling among molecules.