Neutron diffraction on a triple-axis spectrometer and a small-angle neutron scattering instrument is used to study the magnetic phase transition in tetragonal ${\mathrm{Ba}}_2{\mathrm{CuGe}}_2{\mathrm{O}}_7$ at zero magnetic field. In addition to the incommensurate cycloidal antiferromagnetic (AFM) long-range order, we establish that weak incommensurate ferromagnetism (FM) also arises below the transition temperature $T_N$ identified by sharp Bragg peaks close to the $\mathrm{\Gamma }$ point. The intensities of both the incommensurate AFM and FM Bragg peaks vanish abruptly at $T_N$, which is indicative of a weak first-order transition. Above $T_N$, evidence is presented that the magnetic intensity within the tetragonal $(a,b)$ plane is distributed on a ring in momentum space whose radius is determined by the incommensurate wave vector of the cycloidal order. We speculate that the associated soft fluctuations are at the origin of the weak first-order transition in the spirit of a scenario proposed by Brazovskii.

We report inelastic neutron scattering measurements of low-energy ($\hslash \omega \lesssim 10$ meV) magnetic excitations in the “11” system ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$. The spin correlations are two-dimensional (2D) in the superconducting samples at low temperature, but appear much more three-dimensional (3D) when the temperature rises well above $T_c\sim 15$ K, with a clear increase of the (dynamic) spin correlation length perpendicular to the Fe planes. This behavior is extremely unusual; typically, the suppression of thermal fluctuations at low temperature would favor the enhancement of 3D correlations, or even ordering, and the reversion to 2D cannot be naturally explained when only the spin degree of freedom is considered. Our results suggest that the low temperature physics in the 11 system, in particular the evolution of low-energy spin excitations towards superconducting pairing, intrinsically involves changes in orbital correlations.

We report a nontrivial transition in the core structure of vortices in two-band superconductors as a function of interband impurity scattering. We demonstrate that, in addition to singular zeros of the order parameter, the vortices there can acquire a circular nodal line around the singular point in one of the superconducting components. It results in the formation of the peculiar “moat”-like profile in one of the superconducting gaps. The moat-core vortices occur generically in the vicinity of the impurity-induced crossover between $s_{\pm }$ and $s_{++}$ states.

The influence of the Dzyaloshinskii-Moriya interaction (DMI) on stripe domains in perpendicularly magnetized thin ferromagnetic films is theoretically and experimentally investigated. We develop a domain spacing model describing the dependence of the stripe domain width on the magnetic properties of the sample. By including the magnetostatic energy of the domain walls the model correctly describes the transition from Bloch to Néel walls with increasing DMI constant. An approach to determine the magnitude of the DMI constant by fitting the stripe domain width as a function of the effective perpendicular anisotropy of wedge-shaped samples is developed and applied to several ultrathin multilayer samples based on Ni/Fe/Cu(001). The magnitude of the DMI constant arising from Fe/Ni and Ni/Fe interfaces is $0.3\pm 0.14$ meV/atom, indicating that the domain walls are in a pure chiral Néel state. Furthermore, phase diagrams of the skyrmionic bubble domain phase are recorded for two samples with different DMI constants, and by scaling the magnetic field a universal phase diagram for perpendicularly magnetized systems is obtained.

Magnetization reversal of interconnected kagome artificial spin ice was studied by the first-order reversal curve (FORC) technique based on the magneto-optical Kerr effect and magnetoresistance measurements. The magnetization reversal exhibits a distinct sixfold symmetry with the external field orientation. When the field is parallel to one of the nano-bar branches, the domain nucleation/propagation and annihilation processes sensitively depend on the field cycling history and the maximum field applied. When the field is nearly perpendicular to one of the branches, the FORC measurement reveals the magnetic interaction between the Dirac strings and orthogonal branches during the magnetization reversal process. Our results demonstrate that the FORC approach provides a comprehensive framework for understanding the magnetic interaction in the magnetization reversal processes of spin-frustrated systems.

We present a systematic ab initio investigation of the longitudinal and transverse spin relaxation times of magnetic single adatoms deposited on metallic substrates. Our analysis based on time-dependent density functional theory shows that the longitudinal time $T_{\parallel }$ is of order femtosecond while the transverse time $T_{\perp }$ is of order picosecond, i.e. $T_{\perp }\gg T_{\parallel }$. This comes as a consequence of the different energy scales of the corresponding processes: $T_{\parallel }$ involves spin-density excitations of order eV, while $T_{\perp }$ is governed by atomic spin excitations of order meV. Comparison to available inelastic scanning tunneling spectroscopy $dI/dV$ experimental curves shows that the order of magnitude of $T_{\perp }$ agrees well with our results. Regarding $T_{\parallel }$, the time scale calculated here is several orders of magnitude faster than what has been measured up to now; we therefore propose that an ultrafast laser pulse measuring technique is required in order to access the ultrafast spin dynamics described in this paper.

Terahertz (THz) time-domain emission spectroscopy in both transmission and reflection configurations has been employed to understand the THz radiation property and surface properties of tungsten disulfide ($\mathrm{W}{\mathrm{S}}_2$). We observed only one polarization of THz radiation under different polarization of pump beam and a saturation effect with the increasing of pump power. The results are different from that of $\mathrm{Mo}{\mathrm{S}}_2$ based on optical rectification in spite of similar physical and optoelectronic properties of them. The nonlinear optical coefficient calculation based on first-principle method combined with the azimuthal angle dependence of THz radiation implies that THz radiation is insensitive to the azimuthal angle in $\mathrm{W}{\mathrm{S}}_2$. From the pump polarization angle dependence of THz radiation, we find that the contribution due to the nonlinear effect is only 12% approximately. All these suggest the main THz mechanism from $\mathrm{W}{\mathrm{S}}_2$ is due to the surface depletion field induced by the surface states. We also analyzed the surface field features of $\mathrm{W}{\mathrm{S}}_2$ with the maximum surface depletion field of approximate $1.2\times {10}^5\mathrm{V}/\mathrm{cm}$. Fresnel law combined with the dipole radiation model is also used to analyze the angular dependence of THz radiation. The results can not only afford a fundamental THz radiation property of layered materials, but also promote the development of THz devices based on layered materials.

We prepare a triple quantum dot with a separate contact lead to each dot to study Pauli spin blockade in the tunnel-coupled three dots in a row. We measure the tunneling current flowing between the center dot and either the left or right dot with the left and right leads as a common source and the center lead as a drain. In the biased stability diagram, we establish Pauli spin blockade in the respective neighboring dots, with features similarly obtained in double-quantum-dot systems. We further realize Pauli spin blockade with two different conditions by tuning the interdot coupling gates: strong and weak interdot tunnel coupling regimes. In the strong-coupling regime we observe significant suppression of cotunneling through the respective double dots due to Pauli spin blockade. We reveal the influence from the third dot in the triple-dot device on this cotunneling Pauli spin blockade and clarify that the cotunneling Pauli spin blockade is lifted by the resonant coupling of excited states to the third dot level as well as spin exchange of the left and right dots with the adjacent reservoir.

We have studied identification of atomic species in Pt-induced nanowires self-assembled on the Ge(001) surface by combining scanning tunneling microscopy (STM) and atomic force microscopy (AFM). A small number of Sn atoms substituted in the top atomic chains were utilized as references to identify the target atomic species. Force spectroscopy data taken above single atoms on the Sn-substituted nanowires showed that the ratio between the maximum attractive forces above the Sn and the pristine chain atoms exhibited a constant value of 0.86. The obtained ratio was identical to that between Sn and Ge atoms, strongly suggestive that the top ridge of the Pt-induced nanowire was composed of Ge dimers. Our findings also demonstrate that AFM chemical identification method can be used to identify the unknown atomic species on surfaces, regardless of the homogeneity in the atomic composition, which has not been addressed so far.

A variety of new and interesting electronic properties have been predicted in graphene monolayer doped to Van Hove singularities (VHSs) of its density of state. However, tuning the Fermi energy to reach a VHS of graphene by either gating or chemical doping is prohibitively difficult, owing to their large energy distance ($\sim 3$ eV). This difficulty can be easily overcome in twisted bilayer graphene (TBG). By introducing a small twist angle between two adjacent graphene sheets, we are able to generate two low-energy VHSs arbitrarily approaching the Fermi energy. Here, we report experimental studies of electronic properties around the VHSs of a slightly TBG through scanning tunneling microscopy measurements. The split of the VHSs is observed and the spatial symmetry breaking of electronic states around the VHSs is directly visualized. These exotic results provide motivation for further theoretical and experimental studies of graphene systems around the VHSs.

We report first-principles density-functional calculations that elucidate mechanisms of atom manipulation with precise placement of the tip of a noncontact atomic force microscope (AFM). We focus on the vacancy-mediated lateral manipulation of the Si adatom on the Si(111)-($7\times 7$) surface where intriguing adatom movement approaching (approach move) or following (follow move) the tip has been observed experimentally. We identify the diffusion pathways of the adatom from a stable position to another with and without the AFM tip and obtain the corresponding diffusion barriers. We find that a diffusion barrier as high as 1 eV without the AFM tip is reduced by a half eV with the assistance of the tip, indicating the increased feasibility of the adatom motion with the assistance of the AFM tip. More importantly, we find that the energy landscape of the adatom motion is modified drastically with the presence of the AFM tip: The most stable positions without the tip become metastable with the tip, whereas the metastable position becomes the most stable; the metastable position becomes unstable in some cases. We find that all these modifications of the energy landscapes depend on the atom-scale positioning of the AFM tip, inducing the spontaneous adatom move, i.e., the atom manipulation via the AFM tip. The obtained theoretical findings unveil the reason for the approach move and follow move. The underlying physics and chemistry of this atom manipulation are found to be the structural relaxation of the tip apex atom and the subsequent bond formation with the diffusing adatom.

We present a detailed theoretical study of the Drude weight and optical conductivity of 8-$Pmmn$ borophene having tilted anisotropic Dirac cones. We provide exact analytical expressions of $xx$ and $yy$ components of the Drude weight as well as maximum optical conductivity. We also obtain exact analytical expressions of the minimum energy (${\epsilon }_1$) required to trigger the optical transitions and energy (${\epsilon }_2$) needed to attain maximum optical conductivity. We find that the Drude weight and optical conductivity are highly anisotropic as a consequence of the anisotropic Dirac cone. The optical conductivities have a nonmonotonic behavior with photon energy in the regime between ${\epsilon }_1$ and ${\epsilon }_2$, as a result of the tilted parameter $v_t$. The tilted parameter can be extracted by knowing ${\epsilon }_1$ and ${\epsilon }_2$ from optical measurements. The maximum values of the components of the optical conductivity do not depend on the carrier density and the tilted parameter. The product of the maximum values of the anisotropic conductivities has the universal value ${(e^2/4\hslash )}^2$. The tilted anisotropic Dirac cones in 8-$Pmmn$ borophene can be realized by the optical conductivity measurement.

We investigate the effect of both strong and weak potential scattering caused by local impurities and extended (line) defects in an array of Luttinger liquid wires. We find that in both cases a finite range interwire interaction stabilizes the metallic state. Based on calculations of the scaling dimensions of one-particle scattering operators, we construct a phase diagram for low-temperature transport along the array. We find that unlike the local impurity case where only conducting and insulating states are realized (metal-insulator transition driven by interactions), the extended line defects may bring the system into a mixed state where conducting or insulating behavior can be observed depending on bare strength of the scatterer (metal-insulator transition driven by disorder).

We discuss several bosonic topological phases in (3+1) dimensions enriched by a global ${\mathbb{Z}}_2$ symmetry, and gauging the ${\mathbb{Z}}_2$ symmetry. More specifically, following the spirit of the bulk-boundary correspondence, expected to hold in topological phases of matter in general, we consider boundary (surface) field theories and their orbifold. From the surface partition functions, we extract the modular $\mathcal{S}$ and $\mathcal{T}$ matrices and compare them with $(2+1)$-dimensional toplogical phase after dimensional reduction. As a specific example, we discuss topologically ordered phases in $(3+1)$ dimensions described by the BF topological quantum field theories, with abelian exchange statistics between pointlike and looplike quasiparticles. Once the ${\mathbb{Z}}_2$ charge conjugation symmetry is gauged, the ${\mathbb{Z}}_2$ flux becomes non-Abelian excitation. The gauged topological phases we are considering here belong to the quantum double model with non-Abelian group in $(3+1)$ dimensions.

We study a pair of capacitively coupled singlet-triplet spin qubits. We characterize the two-qubit decoherence through two complementary measures, the decay time of coupled-qubit oscillations and the fidelity of entangled state preparation. We provide a quantitative map of their dependence on charge noise and field noise, and we highlight the magnetic field gradient across each singlet-triplet qubit as an effective tool to suppress decoherence due to charge noise.

We use first-principles quantum Monte Carlo simulations to study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between hydrogen adatoms attached to a graphene sheet. We find that the pairwise RKKY interactions at distances of a few lattice spacings are strongly affected by interelectron interactions, in particular, the potential barrier between widely separated adatoms and the dimer configuration becomes wider and thus harder to penetrate. We also point out that antiferrromagnetic and charge density wave orderings have very different effects on the RKKY interaction. Finally, we analyze the stability of several regular adatom superlattices with respect to small displacements of a single adatom, distinguishing the cases of adatoms which populate either both or only one sublattice of the graphene lattice.

Using density functional dynamical mean-field theory (DFDMFT) we address the problem of antiferromagnetic spin ordering in isotropically superstrained graphene. It is shown that the interplay between strain-induced one-particle band narrowing and sizable on-site electron-electron interactions naturally stabilizes a magnetic phase with orbital-selective spin-polarized $p$-band electronic states. While an antiferromagnetic phase with strong local moments arises in the $p_z$ orbitals, the $p_{x,y}$ bands reveal a metallic state with quenched sublattice magnetization. We next investigate the possibility of superconductivity to emerge in this selective magnetoelectronic state. Our theory is expected to be an important step to understanding the next generation of flexible electronics made of Mott localized carbon-based materials as well as the ability of superstrained graphene to host coexisting superconductivity and magnetism at low temperatures.

We study the conductance fluctuation in topological semimetals. Through statistical distribution of energy levels of topological semimetals, we determine the dominant parameters of the universal conductance fluctuation (UCF), i.e., the number of uncorrelated bands $k$, the level degeneracy $s$, and the symmetry parameter $\beta$. These parameters allow us to predict the zero-temperature intrinsic UCF of topological semimetals using the Altshuler-Lee-Stone theory. Then, we obtain numerically the conductance fluctuations for topological semimetals of quasi-one-dimensional geometry. We find that for Dirac (Weyl) semimetals, the theoretical prediction coincides with the numerical results. However, a nonuniversal conductance fluctuation behavior is found for topological nodal line semimetals; that is, the conductance fluctuation amplitude increases with the enlargement of spin-orbit-coupling strength. We find that such unexpected parameter-dependent phenomena of conductance fluctuation are related to the Fermi-surface shape of three-dimensional (3D) topological semimetals. These results will help us to understand the existing and future experimental results of UCF in 3D topological semimetals.