We introduce a method for summing Feynman's perturbation series based on diagrammatic Monte Carlo that significantly improves its convergence properties. This allows us to investigate in a controllable manner the pseudogap regime of the Hubbard model and to study the nodal/antinodal dichotomy at low doping and intermediate coupling. Marked differences from the weak-coupling scenario are manifest, such as a higher degree of incoherence at the antinodes than at the “hot spots”. Our results show that the pseudogap and reduction of quasiparticle coherence at the antinode is due to antiferromagnetic spin correlations centered around the commensurate $(\pi ,\pi )$ wave vector. In contrast, the dominant source of scattering at the node is associated with incommensurate momentum transfer. Umklapp scattering is found to play a key role in the nodal/antinodal dichotomy.

We investigate the nonequilibrium transport properties of a silicene armchair nanoribbon with a random distribution of adsorbed atoms in apex positions. A ferromagnetic insulator grown below the nanoribbon splits spin-up and spin-down electron bands and gives rise to a spin polarization of the conductance. The conductance vanishes when the Fermi energy matches the adatom levels due to the coupling of adatom localized states with the continuum spectra of the nanoribbon. This is the well-known Fano effect, resulting in a spin-dependent antiresonance in the conductance. The different antiresonance energies of spin-up and spin-down electrons give rise to a full spin polarization of the conductance in a broad energy window. This spin-dependent Fano effect opens the possibility to using it in spintronics as a tuneable source of polarized electrons.

Using density functional theory calculations, we establish the hitherto unknown compound ${\mathrm{CrCTe}}_3$ to be a stable antiferromagnetic semiconductor in the $\mathrm{R}\overline{3}$ crystal structure with an indirect fundamental gap. Successive layers in the bulk compound are weakly bound by van der Waals forces so that individual layers can be easily exfoliated. A monolayer of ${\mathrm{CrCTe}}_3$ is also an antiferromagnetic semiconductor. The monolayer is structurally stable over a large range of compressive and tensile strains, and the antiferromagnetic state is robust over this strain range. Band gap of the monolayer can be tuned by as much as 50% by applying strain in this range.

We study the electrocaloric (EC) effect in bulk ${\mathrm{BaTiO}}_3$ (BTO) using molecular dynamics simulations of a first principles-based effective Hamiltonian, combined with direct measurements of the adiabatic EC temperature change in BTO single crystals. We examine in particular the dependence of the EC effect on the direction of the applied electric field at all three ferroelectric transitions, and we show that the EC response is strongly anisotropic. Most strikingly, an inverse caloric effect, i.e., a temperature increase under field removal, can be observed at both ferroelectric-ferroelectric transitions for certain orientations of the applied field. Using the generalized Clausius-Clapeyron equation, we show that the inverse effect occurs exactly for those cases where the field orientation favors the higher temperature/higher entropy phase. Our simulations show that temperature changes of around 1 K can, in principle, be obtained at the tetragonal-orthorhombic transition close to room temperature, even for small applied fields, provided that the applied field is strong enough to drive the system across the first-order transition line. Our direct EC measurements for BTO single crystals at the cubic-tetragonal and at the tetragonal-orthorhombic transitions are in good qualitative agreement with our theoretical predictions, and in particular confirm the occurrence of an inverse EC effect at the tetragonal-orthorhombic transition for electric fields applied along the [001] pseudocubic direction.

Among spin arrangements prone to geometric frustration, the so-called trillium lattice has not been very intensively investigated. A few theoretical works show that it is at the border between a degenerate, an only partially ordered, and a fully ordered ground state. However, only few compounds with this structure have been studied, and there is presently no good example of a trillium lattice with an antiferromagnetic ground state and clear evidence for frustration effects. We present magnetic and specific heat measurements on two realizations of a trillium lattice of local spins, EuPtSi and EuPtGe. Both compounds exhibit a similar magnetic behavior, with ${\mathrm{Eu}}^{2+}$ moments ordering antiferromagnetically at $T_{\mathrm{N}}$=4.1 K (EuPtSi) and 3.3 K (EuPtGe), albeit retaining a considerable amount of entropy in strong magnetic fluctuations extending to temperatures well above $T_{\mathrm{N}}$. The magnetic entropy reaches only roughly half of $Rln8$ at $T_{\mathrm{N}}$. These fluctuations are presumably the source for the pronounced first-order character of the transition at $T_{\mathrm{N}}$ and are likely due to magnetic frustration. Thus, EuPtSi and EuPtGe open a new door to experimental studies of frustration effects in the trillium lattice and provide a testing ground for theoretical predictions.

The low-energy magnetic configurations of artificial, frustrated classical spin chains are investigated using magnetic force microscopy and micromagnetic simulations. Contrary to most studies on two-dimensional artificial spin systems in which frustration arises from the lattice geometry, here magnetic frustration originates from competing interactions between neighboring spins. By tuning continuously the strength and sign of these interactions, we show that different magnetic phases can be stabilized. Comparison between our experimental findings and predictions from the one-dimensional anisotropic next-nearest-neighbor Ising model reveals that artificial frustrated spin chains have a richer phase diagram than initially expected. In addition to the observation of several magnetic orders and the potential extension of this work to highly degenerated artificial spin chains, our results suggest that the micromagnetic nature of the individual magnetic elements allows for the observation of metastable spin configurations.

We report on the nonlocal detection of optically oriented spins in lightly $n$-doped germanium at room temperature. Localized spin generation is achieved by scanning a circularly polarized laser beam ($\lambda =1550$ nm) on an array of lithographically defined Pt microstructures. The in-plane oriented spin generated at the edges of such microstructures, placed at different distances from a spin-detection element, allows for a direct imaging of spin diffusion in the semiconductor, leading to a measured spin diffusion length of about $10\mu \mathrm{m}$. Two different spin-detection blocks are employed, consisting of either a magnetic tunnel junction or a platinum stripe where the spin current is converted in an electrical signal by the inverse spin-Hall effect. The second solution represents the realization of a nonlocal spin-injection/detection scheme that is completely free from ferromagnetic functional blocks.

The Dzyaloshinskii-Moriya interaction (DMI) can cause spin-wave (SW) nonreciprocality in extended magnetic films. Here, we report a correlated effect for SWs propagating in a waveguide, namely, a longitudinally magnetized nanowire. Due to the shape confinement, SWs traveling in the wire are quasiquantized along the width direction. In the presence of the DMI, the wave-front lines become oblique with respect to the propagation direction, unlike any ordinary wave pattern. The tilting direction of the wave front is determined by the sign of the DMI constant, manifesting the chiral nature of the DMI. Another remarkable SW property revealed is that the two dynamical magnetization components that jointly characterize a SW mode exhibit different spatial profiles. Mathematically, these extraordinary effects can be attributed to the first-order spatial derivative brought into the wave equation by the DMI.

Using bulk magnetometry, it was experimentally verified that the metamagnetic insulator-to-metal transition observed in the perovskite manganite ${\mathrm{Pr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ (PCMO) occurs at different critical values of the magnetic-field strength, depending on the macroscopic geometry of the sample in the absence of applied elastic strain. In dimensionally confined samples, such as nanoparticles and thin films, the critical magnetic field can be lower than in bulk by several teslas. To gain a practical understanding of the mechanism behind this correlation, a phenomenological mesoscale simulation was developed to model and analyze the magnetic hysteresis of PCMO. It could be concluded that the low-dimensional samples have a well-defined, permanent free energy bias towards ferromagnetism, something the largely antiferromagnetic bulk all but lacks. The metamagnetic transition field of PCMO turns out to be globally minimized when no external elastic strain is applied and the smallest spatial dimension of the sample approaches 100 nm. The result is likely to apply equally to other low-bandwidth manganites.

Scalar spin chirality, a three-body spin correlation that breaks time-reversal symmetry, is revealed to couple directly to circularly polarized laser. This is shown by the Floquet formalism for the periodically driven repulsive Hubbard model with a strong-coupling expansion. A systematic derivation of the effective low-energy Hamiltonian for a spin degree of freedom reveals that the coupling constant for scalar spin chirality can become significant for a situation in which the driving frequency and the on-site interaction are comparable. This implies that the scalar chirality can be induced by circularly polarized lights, or that it can be used conversely for probing the chirality in Mott insulators as a circular dichroism.

We investigate the penetration depth of high-quality ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ single crystals by a planar waveguide resonator technique, in a cavity perturbation approach. The experimental ${\lambda }_L$ is compared to calculations based on the three-band Eliashberg equations within the $s\pm$ wave model. To this end, the anisotropy of the penetration depth is taken into account. In fact, the agreement between theory and experiment is remarkable. The low-temperature value of the in-plane penetration depth, ${\lambda }_{L,ab}\left(5\text{K}\right)=220\mathrm{nm}$, and the total plasma frequency, ${\omega }_p=1.0\mathrm{eV}$, are also consistent with earlier results. This overall consistency validates the model itself, thus allowing us to estimate parameters that are missing in literature, such as the plasma frequency for each band: it turns out that ${\omega }_{p,1}={\omega }_{p,3}=0.32\mathrm{eV}$ and ${\omega }_{p,2}=0.89\mathrm{eV}$, with the subscripts 1 and 2 denoting the two hole bands and 3 the equivalent electron band.

We have studied new superconductors prepared by metal doping of two-dimensional (2D) layered materials, FeSe and $\mathrm{FeS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$, using ethylenediamine (EDA). The superconducting transition temperatures ($T_c\mathrm{s}$) of metal-doped FeSe and metal-doped $\mathrm{FeS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$, i.e., ${(\mathrm{EDA})}_y{M}_x\mathrm{FeSe}$ and ${\left(\mathrm{EDA}\right)}_y{M}_x\mathrm{FeS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$ ($M$: Li, Na, and K), were 31–45 K and 19–25 K, respectively. The stoichiometry of each sample was clarified by energy dispersive x-ray (EDX) spectroscopy, and the x-ray powder diffraction pattern indicated a large expansion of lattice constant $c$, indicating the cointercalation of metal atoms and EDA. The pressure dependence of superconductivity in ${(\mathrm{EDA})}_y\mathrm{N}{\mathrm{a}}_x\mathrm{FeS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$ has been investigated at a pressure of $0–0.8\mathrm{GPa}$, showing negative pressure dependence in the same manner as $(\mathrm{N}{\mathrm{H}}_3{)}_y\mathrm{N}{\mathrm{a}}_x\mathrm{FeS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$. The $T_c-c$ phase diagrams of $M_x\mathrm{FeSe}$ and $M_x\mathrm{FeS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$ were drawn afresh from the $T_c$ and $c$ of ${(\mathrm{EDA})}_y{M}_x\mathrm{FeSe}$ and ${\left(\mathrm{EDA}\right)}_y{M}_x\mathrm{FeS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$, showing that the $T_c$ increases with increasing $c$ but that extreme expansion of $c$ reverses the $T_c$ trend.

The accuracy of $GW$ in the determination of defect energy levels is assessed through calculations on a set of well-characterized point defects in semiconductors: the As antisite in GaAs, the substitutional Mg in GaN, the interstitial C in Si, the Si dangling bond, and the Si split-vacancy complex in diamond. We show that the $GW$ scheme achieves a reliable description of charge transition levels, but the overall accuracy is comparable to that of hybrid-functional calculations.

Spin-current generation by fluid motion is theoretically investigated. Based on quantum kinetic theory, the spin-diffusion equation coupled with fluid vorticity is derived. We show that spin currents are generated by the vorticity gradient in both laminar and turbulent flows and that the generated spin currents can be detected by the inverse spin Hall voltage measurements, which are predicted to be proportional to the flow velocity in a laminar flow. In contrast, the voltage in a turbulent flow is proportional to the square of the flow velocity. This study will pave the way to fluid spintronics.

Honeycomb iridates such as $\gamma -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ are argued to realize Kitaev spin-anisotropic magnetic exchange, along with Heisenberg and possibly other couplings. While systems with pure Kitaev interactions are candidates to realize a quantum spin-liquid ground state, in $\gamma -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ it has been shown that the presence of competing magnetic interactions leads to an incommensurate spiral spin order at ambient pressure below 38 K. We study the pressure sensitivity of this magnetically ordered state in single crystals of $\gamma -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ using resonant x-ray scattering (RXS) under applied hydrostatic pressures of up to 3 GPa. RXS is a direct probe of electronic order, and we observe the abrupt disappearance of the ${\mathbf{q}}_{\mathrm{sp}}=(0.57,0,0)$ spiral order at a critical pressure $P_c=1.4$ GPa with no accompanying change in the symmetry of the lattice.

The effects of W doping on the structure and optical properties of Aurivillius ferroelectric ${\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{2.5}{\mathrm{Nb}}_2{\mathrm{O}}_9$-based ceramics have been investigated by x-ray diffraction, far-infrared reflectance, and spectroscopic ellipsometry (SE). Moreover, the temperature dependent lattice dynamics and electronic properties have been systemically explored by Raman scattering (77–800 K) and SE (200–800 K), respectively. The anomalous temperature dependence of phonon frequencies around 600 K can be ascribed to the existence of an intermediate phase between ferroelectric orthorhombic and paraelectric tetragonal phases, which is also confirmed by temperature dependent electronic transitions extracted by the SE technique. In addition, the intrinsic evolution mechanism of the structure and electronic transitions during the phase transitions has been clarified by first-principles calculations, which indicate the effective application of noncontact optical methods in judging the phase transformations of ferroelectric materials.

With electron and hole pockets touching at the Weyl node, type-II Weyl semimetal is a newly proposed topological state distinct from its type-I cousin. We numerically study the localization effect for tilted type-I as well as type-II Weyl semimetals and give the global phase diagram. For disordered type-I Weyl semimetal, an intermediate three-dimensional quantum anomalous Hall phase is confirmed between Weyl semimetal phase and diffusive metal phase. However, this intermediate phase is absent for disordered type-II Weyl semimetal. Besides, along the direction of tilt, comparing to its type-I cousin, type-II Weyl semimetal typically possesses longer normalized localization length and therefore it is more robust against disorder. Near the phase boundary between the type-I and the type-II Weyl semimetals, infinitesimal disorder will induce an insulating phase so that, in this region, the concept of Weyl semimetal is meaningless for real materials.

We studied the variation in perpendicular magnetic anisotropy of (111) textured $\mathrm{Au}/N\times \left[\mathrm{Co}/\mathrm{Ni}\right]/\mathrm{Au}$ films as a function of the number of bilayer repeats $N$. The ferromagnetic resonance and superconducting quantum interference device magnetometer measurements show that the perpendicular magnetic anisotropy of Co/Ni multilayers first increases with $N$ for $N\le 10$ and then moderately decreases for $N>10$. The model we propose reveals that the decrease of the anisotropy for $N<10$ is predominantly due to the reduction in the magnetoelastic and magnetocrystalline anisotropies. A moderate decrease in the perpendicular magnetic anisotropy for $N>10$ is due to the reduction in the magnetocrystalline and the surface anisotropies. To calculate the contribution of magnetoelastic anisotropy in the Co/Ni multilayers, in-plane and out-of-plane x-ray diffraction measurements are performed to determine the spacing between Co/Ni (111) and (220) planes. The magnetocrystalline bulk anisotropy is estimated from the difference in the perpendicular and parallel $g$ factors of Co/Ni multilayers that are measured using the in-plane and out-of-plane ferromagnetic resonance measurements. Transmission electron microscopy has been used to estimate the multilayer film roughness. These values are used to calculate the roughness-induced surface and magnetocrystalline anisotropy coefficients as a function of $N$.

${\mathrm{CaCu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ shows a broad maximum at around 200 K in temperature dependence of magnetic susceptibility, whose origin is under debate. The present study addresses this problem, using high-quality samples of ${\mathrm{Ca}}_{1-x}{\mathrm{A}}_x{\mathrm{Cu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ (A = La, Na, and Sr) made by high-pressure synthesis technique. Unlike in a previous report, the maximum shifts to lower temperatures for the La substitution, becomes obscure by the Na substitution, and is less influenced by the Sr substitution. This behavior strongly suggests that the susceptibility maximum is caused by a sharp peak in the density of states just above the Fermi level, which induces strong spin fluctuations. Furthermore, the nature of electronic states of ${\mathrm{LaCu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ and ${\mathrm{NaCu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ are discussed; the former likely bears a Kondo character, and the latter has spin fluctuations of different origin below approximately 150 K.